diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 7646c15..d342959 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake

@@ -294,6 +294,8 @@
   "types/internal/conformance_aliases.h"
   "types/internal/conformance_archetype.h"
   "types/internal/conformance_profile.h"
+  "types/internal/parentheses.h"
+  "types/internal/transform_args.h"
   "types/internal/variant.h"
   "types/optional.h"
   "types/internal/optional.h"

diff --git a/CMake/abslConfig.cmake.in b/CMake/abslConfig.cmake.in
index 60847fa..62d246d 100644
--- a/CMake/abslConfig.cmake.in
+++ b/CMake/abslConfig.cmake.in

@@ -1,7 +1,8 @@
 # absl CMake configuration file.
 
-include(FindThreads)
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
 
 @PACKAGE_INIT@
 
-include ("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
\ No newline at end of file
+include ("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e94dcd3..4767045 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt

@@ -81,6 +81,10 @@
 ## pthread
 find_package(Threads REQUIRED)
 
+option(ABSL_USE_EXTERNAL_GOOGLETEST
+  "If ON, Abseil will assume that the targets for GoogleTest are already provided by the including project. This makes sense when Abseil is used with add_subproject." OFF)
+
+
 option(ABSL_USE_GOOGLETEST_HEAD
   "If ON, abseil will download HEAD from googletest at config time." OFF)
 
@@ -99,14 +103,15 @@
 
 ## check targets
 if(BUILD_TESTING)
-
-  set(absl_gtest_build_dir ${CMAKE_BINARY_DIR}/googletest-build)
-  if(${ABSL_USE_GOOGLETEST_HEAD})
-    set(absl_gtest_src_dir ${CMAKE_BINARY_DIR}/googletest-src)
-  else()
-    set(absl_gtest_src_dir ${ABSL_LOCAL_GOOGLETEST_DIR})
+  if (NOT ABSL_USE_EXTERNAL_GOOGLETEST)
+    set(absl_gtest_build_dir ${CMAKE_BINARY_DIR}/googletest-build)
+    if(${ABSL_USE_GOOGLETEST_HEAD})
+      set(absl_gtest_src_dir ${CMAKE_BINARY_DIR}/googletest-src)
+    else()
+      set(absl_gtest_src_dir ${ABSL_LOCAL_GOOGLETEST_DIR})
+    endif()
+    include(CMake/Googletest/DownloadGTest.cmake)
   endif()
-  include(CMake/Googletest/DownloadGTest.cmake)
 
   check_target(gtest)
   check_target(gtest_main)

diff --git a/absl/BUILD.bazel b/absl/BUILD.bazel
index 5a03acf..f7fc2a7 100644
--- a/absl/BUILD.bazel
+++ b/absl/BUILD.bazel

@@ -44,9 +44,10 @@
 
 config_setting(
     name = "windows",
-    values = {
-        "cpu": "x64_windows",
-    },
+    constraint_values = [
+        "@bazel_tools//platforms:x86_64",
+        "@bazel_tools//platforms:windows",
+    ],
     visibility = [":__subpackages__"],
 )
 

diff --git a/absl/algorithm/BUILD.bazel b/absl/algorithm/BUILD.bazel
index 6a96420..229cd71 100644
--- a/absl/algorithm/BUILD.bazel
+++ b/absl/algorithm/BUILD.bazel

@@ -31,7 +31,9 @@
     hdrs = ["algorithm.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:config"],
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_test(

diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 5454992..a63b591 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt

@@ -191,7 +191,7 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
-    $<$<BOOL:${LIBRT}>:${LIBRT}>
+    $<$<BOOL:${LIBRT}>:-lrt>
     $<$<BOOL:${MINGW}>:"advapi32">
   DEPS
     absl::atomic_hook

diff --git a/absl/base/internal/endian_test.cc b/absl/base/internal/endian_test.cc
index aa6b849..a1691b1 100644
--- a/absl/base/internal/endian_test.cc
+++ b/absl/base/internal/endian_test.cc

@@ -54,24 +54,22 @@
 const uint16_t k16ValueBE{0x2301};
 #endif
 
-template<typename T>
-std::vector<T> GenerateAllValuesForType() {
-  std::vector<T> result;
-  T next = std::numeric_limits<T>::min();
-  while (true) {
-    result.push_back(next);
-    if (next == std::numeric_limits<T>::max()) {
-      return result;
-    }
-    ++next;
+std::vector<uint16_t> GenerateAllUint16Values() {
+  std::vector<uint16_t> result;
+  result.reserve(size_t{1} << (sizeof(uint16_t) * 8));
+  for (uint32_t i = std::numeric_limits<uint16_t>::min();
+       i <= std::numeric_limits<uint16_t>::max(); ++i) {
+    result.push_back(static_cast<uint16_t>(i));
   }
+  return result;
 }
 
 template<typename T>
-std::vector<T> GenerateRandomIntegers(size_t numValuesToTest) {
+std::vector<T> GenerateRandomIntegers(size_t num_values_to_test) {
   std::vector<T> result;
+  result.reserve(num_values_to_test);
   std::mt19937_64 rng(kRandomSeed);
-  for (size_t i = 0; i < numValuesToTest; ++i) {
+  for (size_t i = 0; i < num_values_to_test; ++i) {
     result.push_back(rng());
   }
   return result;
@@ -148,7 +146,7 @@
 }
 
 TEST(EndianessTest, Uint16) {
-  GBSwapHelper(GenerateAllValuesForType<uint16_t>(), &Swap16);
+  GBSwapHelper(GenerateAllUint16Values(), &Swap16);
 }
 
 TEST(EndianessTest, Uint32) {

diff --git a/absl/base/optimization.h b/absl/base/optimization.h
index 646523b..1541d7a 100644
--- a/absl/base/optimization.h
+++ b/absl/base/optimization.h

@@ -178,4 +178,38 @@
 #define ABSL_PREDICT_TRUE(x) (x)
 #endif
 
+// ABSL_INTERNAL_ASSUME(cond)
+// Informs the compiler than a condition is always true and that it can assume
+// it to be true for optimization purposes. The call has undefined behavior if
+// the condition is false.
+// In !NDEBUG mode, the condition is checked with an assert().
+// NOTE: The expression must not have side effects, as it will only be evaluated
+// in some compilation modes and not others.
+//
+// Example:
+//
+//   int x = ...;
+//   ABSL_INTERNAL_ASSUME(x >= 0);
+//   // The compiler can optimize the division to a simple right shift using the
+//   // assumption specified above.
+//   int y = x / 16;
+//
+#if !defined(NDEBUG)
+#define ABSL_INTERNAL_ASSUME(cond) assert(cond)
+#elif ABSL_HAVE_BUILTIN(__builtin_assume)
+#define ABSL_INTERNAL_ASSUME(cond) __builtin_assume(cond)
+#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
+#define ABSL_INTERNAL_ASSUME(cond)        \
+  do {                                    \
+    if (!(cond)) __builtin_unreachable(); \
+  } while (0)
+#elif defined(_MSC_VER)
+#define ABSL_INTERNAL_ASSUME(cond) __assume(cond)
+#else
+#define ABSL_INTERNAL_ASSUME(cond)      \
+  do {                                  \
+    static_cast<void>(false && (cond)); \
+  } while (0)
+#endif
+
 #endif  // ABSL_BASE_OPTIMIZATION_H_

diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 1b0710b..0692123 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel

@@ -73,6 +73,7 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":counting_allocator",
         ":fixed_array",
         "//absl/base:config",
         "//absl/base:exception_testing",
@@ -257,6 +258,7 @@
         ":unordered_map_lookup_test",
         ":unordered_map_members_test",
         ":unordered_map_modifiers_test",
+        "//absl/base:raw_logging_internal",
         "//absl/types:any",
         "@com_google_googletest//:gtest_main",
     ],
@@ -290,6 +292,7 @@
         ":unordered_set_lookup_test",
         ":unordered_set_members_test",
         ":unordered_set_modifiers_test",
+        "//absl/base:raw_logging_internal",
         "//absl/memory",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",

diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index d79fa12..13d9695 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt

@@ -147,6 +147,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::fixed_array
+    absl::counting_allocator
     absl::config
     absl::exception_testing
     absl::hash_testing
@@ -303,6 +304,7 @@
     absl::unordered_map_members_test
     absl::unordered_map_modifiers_test
     absl::any
+    absl::raw_logging_internal
     gmock_main
 )
 
@@ -339,6 +341,7 @@
     absl::unordered_set_members_test
     absl::unordered_set_modifiers_test
     absl::memory
+    absl::raw_logging_internal
     absl::strings
     gmock_main
 )

diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
index 7ccdf6a..bbdb5f4 100644
--- a/absl/container/btree_test.cc
+++ b/absl/container/btree_test.cc

@@ -1543,7 +1543,7 @@
 #ifdef ABSL_HAVE_EXCEPTIONS
   EXPECT_THROW(map.at(3), std::out_of_range);
 #else
-  EXPECT_DEATH(map.at(3), "absl::btree_map::at");
+  EXPECT_DEATH_IF_SUPPORTED(map.at(3), "absl::btree_map::at");
 #endif
 }
 

diff --git a/absl/container/fixed_array.h b/absl/container/fixed_array.h
index 796dd62..adf0dc8 100644
--- a/absl/container/fixed_array.h
+++ b/absl/container/fixed_array.h

@@ -106,13 +106,13 @@
 
  public:
   using allocator_type = typename AllocatorTraits::allocator_type;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;

diff --git a/absl/container/fixed_array_test.cc b/absl/container/fixed_array_test.cc
index 9b1c224..064a88a 100644
--- a/absl/container/fixed_array_test.cc
+++ b/absl/container/fixed_array_test.cc

@@ -29,6 +29,7 @@
 #include "gtest/gtest.h"
 #include "absl/base/internal/exception_testing.h"
 #include "absl/base/options.h"
+#include "absl/container/internal/counting_allocator.h"
 #include "absl/hash/hash_testing.h"
 #include "absl/memory/memory.h"
 
@@ -638,70 +639,9 @@
 }
 #endif  // __GNUC__
 
-// This is a stateful allocator, but the state lives outside of the
-// allocator (in whatever test is using the allocator). This is odd
-// but helps in tests where the allocator is propagated into nested
-// containers - that chain of allocators uses the same state and is
-// thus easier to query for aggregate allocation information.
-template <typename T>
-class CountingAllocator : public std::allocator<T> {
- public:
-  using Alloc = std::allocator<T>;
-  using pointer = typename Alloc::pointer;
-  using size_type = typename Alloc::size_type;
-
-  CountingAllocator() : bytes_used_(nullptr), instance_count_(nullptr) {}
-  explicit CountingAllocator(int64_t* b)
-      : bytes_used_(b), instance_count_(nullptr) {}
-  CountingAllocator(int64_t* b, int64_t* a)
-      : bytes_used_(b), instance_count_(a) {}
-
-  template <typename U>
-  explicit CountingAllocator(const CountingAllocator<U>& x)
-      : Alloc(x),
-        bytes_used_(x.bytes_used_),
-        instance_count_(x.instance_count_) {}
-
-  pointer allocate(size_type n, const void* const hint = nullptr) {
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ += n * sizeof(T);
-    return Alloc::allocate(n, hint);
-  }
-
-  void deallocate(pointer p, size_type n) {
-    Alloc::deallocate(p, n);
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ -= n * sizeof(T);
-  }
-
-  template <typename... Args>
-  void construct(pointer p, Args&&... args) {
-    Alloc::construct(p, absl::forward<Args>(args)...);
-    if (instance_count_) {
-      *instance_count_ += 1;
-    }
-  }
-
-  void destroy(pointer p) {
-    Alloc::destroy(p);
-    if (instance_count_) {
-      *instance_count_ -= 1;
-    }
-  }
-
-  template <typename U>
-  class rebind {
-   public:
-    using other = CountingAllocator<U>;
-  };
-
-  int64_t* bytes_used_;
-  int64_t* instance_count_;
-};
-
 TEST(AllocatorSupportTest, CountInlineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated = 0;
@@ -722,7 +662,7 @@
 
 TEST(AllocatorSupportTest, CountOutoflineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated = 0;
@@ -743,7 +683,7 @@
 
 TEST(AllocatorSupportTest, CountCopyInlineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated1 = 0;
@@ -771,7 +711,7 @@
 
 TEST(AllocatorSupportTest, CountCopyOutoflineAllocations) {
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   int64_t allocated1 = 0;
@@ -803,7 +743,7 @@
   using testing::SizeIs;
 
   constexpr size_t inlined_size = 4;
-  using Alloc = CountingAllocator<int>;
+  using Alloc = absl::container_internal::CountingAllocator<int>;
   using AllocFxdArr = absl::FixedArray<int, inlined_size, Alloc>;
 
   {
@@ -833,10 +773,10 @@
   int* raw = a.data();
   raw[0] = 0;
   raw[9] = 0;
-  EXPECT_DEATH(raw[-2] = 0, "container-overflow");
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[10] = 0, "container-overflow");
-  EXPECT_DEATH(raw[31] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-2] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[10] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[31] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations2) {
@@ -844,10 +784,10 @@
   char* raw = a.data();
   raw[0] = 0;
   raw[11] = 0;
-  EXPECT_DEATH(raw[-7] = 0, "container-overflow");
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[12] = 0, "container-overflow");
-  EXPECT_DEATH(raw[17] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-7] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[12] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[17] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations3) {
@@ -855,8 +795,8 @@
   uint64_t* raw = a.data();
   raw[0] = 0;
   raw[19] = 0;
-  EXPECT_DEATH(raw[-1] = 0, "container-overflow");
-  EXPECT_DEATH(raw[20] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1] = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[20] = 0, "container-overflow");
 }
 
 TEST(FixedArrayTest, AddressSanitizerAnnotations4) {
@@ -868,11 +808,11 @@
   // there is only a 8-byte red zone before the container range, so we only
   // access the last 4 bytes of the struct to make sure it stays within the red
   // zone.
-  EXPECT_DEATH(raw[-1].z_ = 0, "container-overflow");
-  EXPECT_DEATH(raw[10] = ThreeInts(), "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[-1].z_ = 0, "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[10] = ThreeInts(), "container-overflow");
   // The actual size of storage is kDefaultBytes=256, 21*12 = 252,
   // so reading raw[21] should still trigger the correct warning.
-  EXPECT_DEATH(raw[21] = ThreeInts(), "container-overflow");
+  EXPECT_DEATH_IF_SUPPORTED(raw[21] = ThreeInts(), "container-overflow");
 }
 #endif  // ADDRESS_SANITIZER
 

diff --git a/absl/container/flat_hash_map_test.cc b/absl/container/flat_hash_map_test.cc
index 728b693..2823c32 100644
--- a/absl/container/flat_hash_map_test.cc
+++ b/absl/container/flat_hash_map_test.cc

@@ -16,6 +16,7 @@
 
 #include <memory>
 
+#include "absl/base/internal/raw_logging.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/unordered_map_constructor_test.h"
 #include "absl/container/internal/unordered_map_lookup_test.h"
@@ -34,6 +35,19 @@
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
+// Check that absl::flat_hash_map works in a global constructor.
+struct BeforeMain {
+  BeforeMain() {
+    absl::flat_hash_map<int, int> x;
+    x.insert({1, 1});
+    ABSL_RAW_CHECK(x.find(0) == x.end(), "x should not contain 0");
+    auto it = x.find(1);
+    ABSL_RAW_CHECK(it != x.end(), "x should contain 1");
+    ABSL_RAW_CHECK(it->second, "1 should map to 1");
+  }
+};
+const BeforeMain before_main;
+
 template <class K, class V>
 using Map = flat_hash_map<K, V, StatefulTestingHash, StatefulTestingEqual,
                           Alloc<std::pair<const K, V>>>;

diff --git a/absl/container/flat_hash_set_test.cc b/absl/container/flat_hash_set_test.cc
index 40d7f85..8f6f994 100644
--- a/absl/container/flat_hash_set_test.cc
+++ b/absl/container/flat_hash_set_test.cc

@@ -16,6 +16,7 @@
 
 #include <vector>
 
+#include "absl/base/internal/raw_logging.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/unordered_set_constructor_test.h"
 #include "absl/container/internal/unordered_set_lookup_test.h"
@@ -36,6 +37,17 @@
 using ::testing::UnorderedElementsAre;
 using ::testing::UnorderedElementsAreArray;
 
+// Check that absl::flat_hash_set works in a global constructor.
+struct BeforeMain {
+  BeforeMain() {
+    absl::flat_hash_set<int> x;
+    x.insert(1);
+    ABSL_RAW_CHECK(!x.contains(0), "x should not contain 0");
+    ABSL_RAW_CHECK(x.contains(1), "x should contain 1");
+  }
+};
+const BeforeMain before_main;
+
 template <class T>
 using Set =
     absl::flat_hash_set<T, StatefulTestingHash, StatefulTestingEqual, Alloc<T>>;

diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index 5f6f615..f18dd4c 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h

@@ -351,14 +351,14 @@
   // Returns a `reference` to the first element of the inlined vector.
   reference front() {
     ABSL_HARDENING_ASSERT(!empty());
-    return at(0);
+    return data()[0];
   }
 
   // Overload of `InlinedVector::front()` that returns a `const_reference` to
   // the first element of the inlined vector.
   const_reference front() const {
     ABSL_HARDENING_ASSERT(!empty());
-    return at(0);
+    return data()[0];
   }
 
   // `InlinedVector::back()`
@@ -366,14 +366,14 @@
   // Returns a `reference` to the last element of the inlined vector.
   reference back() {
     ABSL_HARDENING_ASSERT(!empty());
-    return at(size() - 1);
+    return data()[size() - 1];
   }
 
   // Overload of `InlinedVector::back()` that returns a `const_reference` to the
   // last element of the inlined vector.
   const_reference back() const {
     ABSL_HARDENING_ASSERT(!empty());
-    return at(size() - 1);
+    return data()[size() - 1];
   }
 
   // `InlinedVector::begin()`
@@ -524,7 +524,7 @@
   void assign(InputIterator first, InputIterator last) {
     size_type i = 0;
     for (; i < size() && first != last; ++i, static_cast<void>(++first)) {
-      at(i) = *first;
+      data()[i] = *first;
     }
 
     erase(data() + i, data() + size());
@@ -535,9 +535,12 @@
   //
   // Resizes the inlined vector to contain `n` elements.
   //
-  // NOTE: if `n` is smaller than `size()`, extra elements are destroyed. If `n`
+  // NOTE: If `n` is smaller than `size()`, extra elements are destroyed. If `n`
   // is larger than `size()`, new elements are value-initialized.
-  void resize(size_type n) { storage_.Resize(DefaultValueAdapter(), n); }
+  void resize(size_type n) {
+    ABSL_HARDENING_ASSERT(n <= max_size());
+    storage_.Resize(DefaultValueAdapter(), n);
+  }
 
   // Overload of `InlinedVector::resize(...)` that resizes the inlined vector to
   // contain `n` elements.
@@ -545,6 +548,7 @@
   // NOTE: if `n` is smaller than `size()`, extra elements are destroyed. If `n`
   // is larger than `size()`, new elements are copied-constructed from `v`.
   void resize(size_type n, const_reference v) {
+    ABSL_HARDENING_ASSERT(n <= max_size());
     storage_.Resize(CopyValueAdapter(v), n);
   }
 

diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index 4504e9c..b23138f 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h

@@ -929,9 +929,15 @@
   void decrement_slow();
 
  public:
+  bool operator==(const iterator &other) const {
+    return node == other.node && position == other.position;
+  }
   bool operator==(const const_iterator &other) const {
     return node == other.node && position == other.position;
   }
+  bool operator!=(const iterator &other) const {
+    return node != other.node || position != other.position;
+  }
   bool operator!=(const const_iterator &other) const {
     return node != other.node || position != other.position;
   }

diff --git a/absl/container/internal/counting_allocator.h b/absl/container/internal/counting_allocator.h
index 9efdc66..927cf08 100644
--- a/absl/container/internal/counting_allocator.h
+++ b/absl/container/internal/counting_allocator.h

@@ -15,7 +15,6 @@
 #ifndef ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
 #define ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
 
-#include <cassert>
 #include <cstdint>
 #include <memory>
 
@@ -31,33 +30,63 @@
 // containers - that chain of allocators uses the same state and is
 // thus easier to query for aggregate allocation information.
 template <typename T>
-class CountingAllocator : public std::allocator<T> {
+class CountingAllocator {
  public:
-  using Alloc = std::allocator<T>;
-  using pointer = typename Alloc::pointer;
-  using size_type = typename Alloc::size_type;
+  using Allocator = std::allocator<T>;
+  using AllocatorTraits = std::allocator_traits<Allocator>;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
 
-  CountingAllocator() : bytes_used_(nullptr) {}
-  explicit CountingAllocator(int64_t* b) : bytes_used_(b) {}
+  CountingAllocator() = default;
+  explicit CountingAllocator(int64_t* bytes_used) : bytes_used_(bytes_used) {}
+  CountingAllocator(int64_t* bytes_used, int64_t* instance_count)
+      : bytes_used_(bytes_used), instance_count_(instance_count) {}
 
   template <typename U>
   CountingAllocator(const CountingAllocator<U>& x)
-      : Alloc(x), bytes_used_(x.bytes_used_) {}
+      : bytes_used_(x.bytes_used_), instance_count_(x.instance_count_) {}
 
-  pointer allocate(size_type n,
-                   std::allocator<void>::const_pointer hint = nullptr) {
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ += n * sizeof(T);
-    return Alloc::allocate(n, hint);
+  pointer allocate(
+      size_type n,
+      typename AllocatorTraits::const_void_pointer hint = nullptr) {
+    Allocator allocator;
+    pointer ptr = AllocatorTraits::allocate(allocator, n, hint);
+    if (bytes_used_ != nullptr) {
+      *bytes_used_ += n * sizeof(T);
+    }
+    return ptr;
   }
 
   void deallocate(pointer p, size_type n) {
-    Alloc::deallocate(p, n);
-    assert(bytes_used_ != nullptr);
-    *bytes_used_ -= n * sizeof(T);
+    Allocator allocator;
+    AllocatorTraits::deallocate(allocator, p, n);
+    if (bytes_used_ != nullptr) {
+      *bytes_used_ -= n * sizeof(T);
+    }
   }
 
-  template<typename U>
+  template <typename U, typename... Args>
+  void construct(U* p, Args&&... args) {
+    Allocator allocator;
+    AllocatorTraits::construct(allocator, p, std::forward<Args>(args)...);
+    if (instance_count_ != nullptr) {
+      *instance_count_ += 1;
+    }
+  }
+
+  template <typename U>
+  void destroy(U* p) {
+    Allocator allocator;
+    AllocatorTraits::destroy(allocator, p);
+    if (instance_count_ != nullptr) {
+      *instance_count_ -= 1;
+    }
+  }
+
+  template <typename U>
   class rebind {
    public:
     using other = CountingAllocator<U>;
@@ -65,7 +94,8 @@
 
   friend bool operator==(const CountingAllocator& a,
                          const CountingAllocator& b) {
-    return a.bytes_used_ == b.bytes_used_;
+    return a.bytes_used_ == b.bytes_used_ &&
+           a.instance_count_ == b.instance_count_;
   }
 
   friend bool operator!=(const CountingAllocator& a,
@@ -73,7 +103,8 @@
     return !(a == b);
   }
 
-  int64_t* bytes_used_;
+  int64_t* bytes_used_ = nullptr;
+  int64_t* instance_count_ = nullptr;
 };
 
 }  // namespace container_internal

diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index 8aaffc3..308119c 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h

@@ -184,11 +184,6 @@
 #error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
 #endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 
-#if (ABSL_PER_THREAD_TLS == 1) && !defined(ABSL_BUILD_DLL) && \
-    !defined(ABSL_CONSUME_DLL)
-#define ABSL_INTERNAL_HASHTABLEZ_SAMPLE
-#endif
-
 #if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
 #endif  // ABSL_PER_THREAD_TLS

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index e47e1fe..df0f2b2 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h

@@ -104,7 +104,7 @@
 
 #include "absl/base/internal/bits.h"
 #include "absl/base/internal/endian.h"
-#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 #include "absl/container/internal/common.h"
 #include "absl/container/internal/compressed_tuple.h"
@@ -649,24 +649,26 @@
     }
 
    private:
-    iterator(ctrl_t* ctrl) : ctrl_(ctrl) {}  // for end()
-    iterator(ctrl_t* ctrl, slot_type* slot) : ctrl_(ctrl), slot_(slot) {}
+    iterator(ctrl_t* ctrl, slot_type* slot) : ctrl_(ctrl), slot_(slot) {
+      // This assumption helps the compiler know that any non-end iterator is
+      // not equal to any end iterator.
+      ABSL_INTERNAL_ASSUME(ctrl != nullptr);
+    }
 
-    void assert_is_full() const { ABSL_HARDENING_ASSERT(IsFull(*ctrl_)); }
+    void assert_is_full() const {
+      ABSL_HARDENING_ASSERT(ctrl_ != nullptr && IsFull(*ctrl_));
+    }
     void assert_is_valid() const {
-      ABSL_HARDENING_ASSERT(!ctrl_ || IsFull(*ctrl_) || *ctrl_ == kSentinel);
+      ABSL_HARDENING_ASSERT(ctrl_ == nullptr || IsFull(*ctrl_));
     }
 
     void skip_empty_or_deleted() {
       while (IsEmptyOrDeleted(*ctrl_)) {
-        // ctrl is not necessarily aligned to Group::kWidth. It is also likely
-        // to read past the space for ctrl bytes and into slots. This is ok
-        // because ctrl has sizeof() == 1 and slot has sizeof() >= 1 so there
-        // is no way to read outside the combined slot array.
         uint32_t shift = Group{ctrl_}.CountLeadingEmptyOrDeleted();
         ctrl_ += shift;
         slot_ += shift;
       }
+      if (ABSL_PREDICT_FALSE(*ctrl_ == kSentinel)) ctrl_ = nullptr;
     }
 
     ctrl_t* ctrl_ = nullptr;
@@ -908,12 +910,12 @@
     it.skip_empty_or_deleted();
     return it;
   }
-  iterator end() { return {ctrl_ + capacity_}; }
+  iterator end() { return {}; }
 
   const_iterator begin() const {
     return const_cast<raw_hash_set*>(this)->begin();
   }
-  const_iterator end() const { return const_cast<raw_hash_set*>(this)->end(); }
+  const_iterator end() const { return {}; }
   const_iterator cbegin() const { return begin(); }
   const_iterator cend() const { return end(); }
 

diff --git a/absl/copts/AbseilConfigureCopts.cmake b/absl/copts/AbseilConfigureCopts.cmake
index 9557e36..acd46d0 100644
--- a/absl/copts/AbseilConfigureCopts.cmake
+++ b/absl/copts/AbseilConfigureCopts.cmake

@@ -12,7 +12,7 @@
   set(ABSL_BUILD_DLL FALSE)
 endif()
 
-if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "AMD64")
+if("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "x86_64|amd64|AMD64")
   if (MSVC)
     set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
   else()

diff --git a/absl/debugging/failure_signal_handler_test.cc b/absl/debugging/failure_signal_handler_test.cc
index 863fb51..d8283b2 100644
--- a/absl/debugging/failure_signal_handler_test.cc
+++ b/absl/debugging/failure_signal_handler_test.cc

@@ -55,7 +55,7 @@
               exit_regex);
 #else
   // Windows doesn't have testing::KilledBySignal().
-  EXPECT_DEATH(InstallHandlerAndRaise(signo), exit_regex);
+  EXPECT_DEATH_IF_SUPPORTED(InstallHandlerAndRaise(signo), exit_regex);
 #endif
 }
 
@@ -107,8 +107,8 @@
               testing::KilledBySignal(signo), exit_regex);
 #else
   // Windows doesn't have testing::KilledBySignal().
-  EXPECT_DEATH(InstallHandlerWithWriteToFileAndRaise(file.c_str(), signo),
-               exit_regex);
+  EXPECT_DEATH_IF_SUPPORTED(
+      InstallHandlerWithWriteToFileAndRaise(file.c_str(), signo), exit_regex);
 #endif
 
   // Open the file in this process and check its contents.

diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 4b51d9d..685e395 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel

@@ -326,7 +326,9 @@
         ":handle",
         ":registry",
         "//absl/base:core_headers",
+        "//absl/base:malloc_internal",
         "//absl/strings",
+        "//absl/time",
         "@com_google_googletest//:gtest_main",
     ],
 )

diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index 2204b0f..ec82ee1 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt

@@ -304,6 +304,7 @@
     absl::flags_internal
     absl::flags_registry
     absl::strings
+    absl::time
     gtest_main
 )
 

diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index bb91765..194a9d3 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h

@@ -149,9 +149,6 @@
   }
   T Get() const { return GetImpl()->Get(); }
   void Set(const T& v) { GetImpl()->Set(v); }
-  void SetCallback(const flags_internal::FlagCallbackFunc mutation_callback) {
-    GetImpl()->SetCallback(mutation_callback);
-  }
   void InvokeCallback() { GetImpl()->InvokeCallback(); }
 
   // The data members are logically private, but they need to be public for
@@ -314,9 +311,9 @@
     static std::string NonConst() { return ABSL_FLAG_IMPL_FLAGHELP(txt); } \
   }
 
-#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
-  static void* AbslFlagsInitFlag##name() {                                  \
-    return absl::flags_internal::MakeFromDefaultValue<Type>(default_value); \
+#define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
+  static void AbslFlagsInitFlag##name(void* dst) {                        \
+    absl::flags_internal::MakeFromDefaultValue<Type>(dst, default_value); \
   }
 
 // ABSL_FLAG_IMPL
@@ -333,8 +330,9 @@
       ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),    \
       absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>(0),   \
       &AbslFlagsInitFlag##name};                                    \
-  extern bool FLAGS_no##name;                                       \
-  bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+  extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name;   \
+  absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name =         \
+      ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
 #else
 // MSVC version uses aggregate initialization. We also do not try to
 // optimize away help wrapper.
@@ -345,8 +343,9 @@
   ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                      \
       ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),      \
       &AbslFlagHelpGenFor##name::NonConst, &AbslFlagsInitFlag##name}; \
-  extern bool FLAGS_no##name;                                         \
-  bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+  extern absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name;     \
+  absl::flags_internal::FlagRegistrarEmpty FLAGS_no##name =           \
+      ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
 #endif
 
 // ABSL_RETIRED_FLAG

diff --git a/absl/flags/flag_test.cc b/absl/flags/flag_test.cc
index 377e3b2..6fa178f 100644
--- a/absl/flags/flag_test.cc
+++ b/absl/flags/flag_test.cc

@@ -19,6 +19,7 @@
 
 #include <cmath>
 #include <string>
+#include <thread>  // NOLINT
 #include <vector>
 
 #include "gtest/gtest.h"
@@ -34,6 +35,7 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
+#include "absl/time/time.h"
 
 ABSL_DECLARE_FLAG(int64_t, mistyped_int_flag);
 ABSL_DECLARE_FLAG(std::vector<std::string>, mistyped_string_flag);
@@ -44,8 +46,8 @@
 
 std::string TestHelpMsg() { return "dynamic help"; }
 template <typename T>
-void* TestMakeDflt() {
-  return new T{};
+void TestMakeDflt(void* dst) {
+  new (dst) T{};
 }
 void TestCallback() {}
 
@@ -74,6 +76,7 @@
 #endif
     return std::string(fname);
   }
+  flags::FlagSaver flag_saver_;
 };
 
 struct S1 {
@@ -107,15 +110,15 @@
             flags::FlagValueStorageKind::kTwoWordsAtomic);
 #else
   EXPECT_EQ(flags::StorageKind<S1>(),
-            flags::FlagValueStorageKind::kHeapAllocated);
+            flags::FlagValueStorageKind::kAlignedBuffer);
   EXPECT_EQ(flags::StorageKind<S2>(),
-            flags::FlagValueStorageKind::kHeapAllocated);
+            flags::FlagValueStorageKind::kAlignedBuffer);
 #endif
 
   EXPECT_EQ(flags::StorageKind<std::string>(),
-            flags::FlagValueStorageKind::kHeapAllocated);
+            flags::FlagValueStorageKind::kAlignedBuffer);
   EXPECT_EQ(flags::StorageKind<std::vector<std::string>>(),
-            flags::FlagValueStorageKind::kHeapAllocated);
+            flags::FlagValueStorageKind::kAlignedBuffer);
 }
 
 // --------------------------------------------------------------------
@@ -190,6 +193,7 @@
 ABSL_DECLARE_FLAG(double, test_flag_09);
 ABSL_DECLARE_FLAG(float, test_flag_10);
 ABSL_DECLARE_FLAG(std::string, test_flag_11);
+ABSL_DECLARE_FLAG(absl::Duration, test_flag_12);
 
 namespace {
 
@@ -208,6 +212,7 @@
   EXPECT_EQ(FLAGS_test_flag_09.Name(), "test_flag_09");
   EXPECT_EQ(FLAGS_test_flag_10.Name(), "test_flag_10");
   EXPECT_EQ(FLAGS_test_flag_11.Name(), "test_flag_11");
+  EXPECT_EQ(FLAGS_test_flag_12.Name(), "test_flag_12");
 }
 #endif  // !ABSL_FLAGS_STRIP_NAMES
 
@@ -226,6 +231,7 @@
 ABSL_FLAG(double, test_flag_09, -9.876e-50, "test flag 09");
 ABSL_FLAG(float, test_flag_10, 1.234e12f, "test flag 10");
 ABSL_FLAG(std::string, test_flag_11, "", "test flag 11");
+ABSL_FLAG(absl::Duration, test_flag_12, absl::Minutes(10), "test flag 12");
 
 namespace {
 
@@ -287,6 +293,11 @@
   EXPECT_EQ(FLAGS_test_flag_11.Help(), "test flag 11");
   EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_11.Filename(), expected_file_name))
       << FLAGS_test_flag_11.Filename();
+
+  EXPECT_EQ(FLAGS_test_flag_12.Name(), "test_flag_12");
+  EXPECT_EQ(FLAGS_test_flag_12.Help(), "test flag 12");
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_12.Filename(), expected_file_name))
+      << FLAGS_test_flag_12.Filename();
 }
 #endif  // !ABSL_FLAGS_STRIP_NAMES
 
@@ -304,6 +315,20 @@
   EXPECT_EQ(FLAGS_test_flag_09.DefaultValue(), "-9.876e-50");
   EXPECT_EQ(FLAGS_test_flag_10.DefaultValue(), "1.234e+12");
   EXPECT_EQ(FLAGS_test_flag_11.DefaultValue(), "");
+  EXPECT_EQ(FLAGS_test_flag_12.DefaultValue(), "10m");
+
+  EXPECT_EQ(FLAGS_test_flag_01.CurrentValue(), "true");
+  EXPECT_EQ(FLAGS_test_flag_02.CurrentValue(), "1234");
+  EXPECT_EQ(FLAGS_test_flag_03.CurrentValue(), "-34");
+  EXPECT_EQ(FLAGS_test_flag_04.CurrentValue(), "189");
+  EXPECT_EQ(FLAGS_test_flag_05.CurrentValue(), "10765");
+  EXPECT_EQ(FLAGS_test_flag_06.CurrentValue(), "40000");
+  EXPECT_EQ(FLAGS_test_flag_07.CurrentValue(), "-1234567");
+  EXPECT_EQ(FLAGS_test_flag_08.CurrentValue(), "9876543");
+  EXPECT_EQ(FLAGS_test_flag_09.CurrentValue(), "-9.876e-50");
+  EXPECT_EQ(FLAGS_test_flag_10.CurrentValue(), "1.234e+12");
+  EXPECT_EQ(FLAGS_test_flag_11.CurrentValue(), "");
+  EXPECT_EQ(FLAGS_test_flag_12.CurrentValue(), "10m");
 
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), true);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_02), 1234);
@@ -316,6 +341,7 @@
   EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_09), -9.876e-50, 1e-55);
   EXPECT_NEAR(absl::GetFlag(FLAGS_test_flag_10), 1.234e12f, 1e5f);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Minutes(10));
 }
 
 // --------------------------------------------------------------------
@@ -408,6 +434,38 @@
 
   absl::SetFlag(&FLAGS_test_flag_11, "asdf");
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_11), "asdf");
+
+  absl::SetFlag(&FLAGS_test_flag_12, absl::Seconds(110));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), absl::Seconds(110));
+}
+
+// --------------------------------------------------------------------
+
+TEST_F(FlagTest, TestGetViaReflection) {
+  auto* handle = flags::FindCommandLineFlag("test_flag_01");
+  EXPECT_EQ(*handle->Get<bool>(), true);
+  handle = flags::FindCommandLineFlag("test_flag_02");
+  EXPECT_EQ(*handle->Get<int>(), 1234);
+  handle = flags::FindCommandLineFlag("test_flag_03");
+  EXPECT_EQ(*handle->Get<int16_t>(), -34);
+  handle = flags::FindCommandLineFlag("test_flag_04");
+  EXPECT_EQ(*handle->Get<uint16_t>(), 189);
+  handle = flags::FindCommandLineFlag("test_flag_05");
+  EXPECT_EQ(*handle->Get<int32_t>(), 10765);
+  handle = flags::FindCommandLineFlag("test_flag_06");
+  EXPECT_EQ(*handle->Get<uint32_t>(), 40000);
+  handle = flags::FindCommandLineFlag("test_flag_07");
+  EXPECT_EQ(*handle->Get<int64_t>(), -1234567);
+  handle = flags::FindCommandLineFlag("test_flag_08");
+  EXPECT_EQ(*handle->Get<uint64_t>(), 9876543);
+  handle = flags::FindCommandLineFlag("test_flag_09");
+  EXPECT_NEAR(*handle->Get<double>(), -9.876e-50, 1e-55);
+  handle = flags::FindCommandLineFlag("test_flag_10");
+  EXPECT_NEAR(*handle->Get<float>(), 1.234e12f, 1e5f);
+  handle = flags::FindCommandLineFlag("test_flag_11");
+  EXPECT_EQ(*handle->Get<std::string>(), "");
+  handle = flags::FindCommandLineFlag("test_flag_12");
+  EXPECT_EQ(*handle->Get<absl::Duration>(), absl::Minutes(10));
 }
 
 // --------------------------------------------------------------------
@@ -416,28 +474,32 @@
 
 }  // namespace
 
-ABSL_FLAG(int, test_flag_12, GetDflt1(), "test flag 12");
-ABSL_FLAG(std::string, test_flag_13, absl::StrCat("AAA", "BBB"),
-          "test flag 13");
+ABSL_FLAG(int, test_int_flag_with_non_const_default, GetDflt1(),
+          "test int flag non const default");
+ABSL_FLAG(std::string, test_string_flag_with_non_const_default,
+          absl::StrCat("AAA", "BBB"), "test string flag non const default");
 
 namespace {
 
 TEST_F(FlagTest, TestNonConstexprDefault) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), 1);
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), "AAABBB");
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_int_flag_with_non_const_default), 1);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_string_flag_with_non_const_default),
+            "AAABBB");
 }
 
 // --------------------------------------------------------------------
 
 }  // namespace
 
-ABSL_FLAG(bool, test_flag_14, true, absl::StrCat("test ", "flag ", "14"));
+ABSL_FLAG(bool, test_flag_with_non_const_help, true,
+          absl::StrCat("test ", "flag ", "non const help"));
 
 namespace {
 
 #if !ABSL_FLAGS_STRIP_HELP
 TEST_F(FlagTest, TestNonConstexprHelp) {
-  EXPECT_EQ(FLAGS_test_flag_14.Help(), "test flag 14");
+  EXPECT_EQ(FLAGS_test_flag_with_non_const_help.Help(),
+            "test flag non const help");
 }
 #endif  //! ABSL_FLAGS_STRIP_HELP
 
@@ -503,14 +565,14 @@
 
 }  // namespace
 
-ABSL_FLAG(CustomUDT, test_flag_15, CustomUDT(), "test flag 15");
+ABSL_FLAG(CustomUDT, test_flag_custom_udt, CustomUDT(), "test flag custom UDT");
 
 namespace {
 
 TEST_F(FlagTest, TestCustomUDT) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(1, 1));
-  absl::SetFlag(&FLAGS_test_flag_15, CustomUDT(2, 3));
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(2, 3));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_custom_udt), CustomUDT(1, 1));
+  absl::SetFlag(&FLAGS_test_flag_custom_udt, CustomUDT(2, 3));
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_custom_udt), CustomUDT(2, 3));
 }
 
 // MSVC produces link error on the type mismatch.
@@ -521,18 +583,21 @@
 
 TEST_F(FlagDeathTest, TestTypeMismatchValidations) {
 #if !defined(NDEBUG)
-  EXPECT_DEATH(static_cast<void>(absl::GetFlag(FLAGS_mistyped_int_flag)),
-               "Flag 'mistyped_int_flag' is defined as one type and declared "
-               "as another");
-  EXPECT_DEATH(static_cast<void>(absl::GetFlag(FLAGS_mistyped_string_flag)),
-               "Flag 'mistyped_string_flag' is defined as one type and "
-               "declared as another");
+  EXPECT_DEATH_IF_SUPPORTED(
+      static_cast<void>(absl::GetFlag(FLAGS_mistyped_int_flag)),
+      "Flag 'mistyped_int_flag' is defined as one type and declared "
+      "as another");
+  EXPECT_DEATH_IF_SUPPORTED(
+      static_cast<void>(absl::GetFlag(FLAGS_mistyped_string_flag)),
+      "Flag 'mistyped_string_flag' is defined as one type and "
+      "declared as another");
 #endif
 
-  EXPECT_DEATH(absl::SetFlag(&FLAGS_mistyped_int_flag, 1),
-               "Flag 'mistyped_int_flag' is defined as one type and declared "
-               "as another");
-  EXPECT_DEATH(
+  EXPECT_DEATH_IF_SUPPORTED(
+      absl::SetFlag(&FLAGS_mistyped_int_flag, 1),
+      "Flag 'mistyped_int_flag' is defined as one type and declared "
+      "as another");
+  EXPECT_DEATH_IF_SUPPORTED(
       absl::SetFlag(&FLAGS_mistyped_string_flag, std::vector<std::string>{}),
       "Flag 'mistyped_string_flag' is defined as one type and declared as "
       "another");
@@ -570,16 +635,17 @@
 
 // Flag default values can be specified with a value that converts to the flag
 // value type implicitly.
-ABSL_FLAG(ConversionTestVal, test_flag_16,
-          ConversionTestVal::ViaImplicitConv::kTen, "test flag 16");
+ABSL_FLAG(ConversionTestVal, test_flag_implicit_conv,
+          ConversionTestVal::ViaImplicitConv::kTen,
+          "test flag init via implicit conversion");
 
 namespace {
 
 TEST_F(FlagTest, CanSetViaImplicitConversion) {
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_16).a, 10);
-  absl::SetFlag(&FLAGS_test_flag_16,
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_implicit_conv).a, 10);
+  absl::SetFlag(&FLAGS_test_flag_implicit_conv,
                 ConversionTestVal::ViaImplicitConv::kEleven);
-  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_16).a, 11);
+  EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_implicit_conv).a, 11);
 }
 
 // --------------------------------------------------------------------
@@ -646,3 +712,69 @@
 }
 
 }  // namespace
+
+// --------------------------------------------------------------------
+
+namespace {
+
+// User-defined type with small alignment, but size exceeding 16.
+struct SmallAlignUDT {
+  SmallAlignUDT() : c('A'), s(12) {}
+  char c;
+  int16_t s;
+  char bytes[14];
+};
+
+bool AbslParseFlag(absl::string_view, SmallAlignUDT*, std::string*) {
+  return true;
+}
+std::string AbslUnparseFlag(const SmallAlignUDT&) { return ""; }
+
+// User-defined type with small size, but not trivially copyable.
+struct NonTriviallyCopyableUDT {
+  NonTriviallyCopyableUDT() : c('A') {}
+  NonTriviallyCopyableUDT(const NonTriviallyCopyableUDT& rhs) : c(rhs.c) {}
+  NonTriviallyCopyableUDT& operator=(const NonTriviallyCopyableUDT& rhs) {
+    c = rhs.c;
+    return *this;
+  }
+
+  char c;
+};
+
+bool AbslParseFlag(absl::string_view, NonTriviallyCopyableUDT*, std::string*) {
+  return true;
+}
+std::string AbslUnparseFlag(const NonTriviallyCopyableUDT&) { return ""; }
+
+}  // namespace
+
+ABSL_FLAG(SmallAlignUDT, test_flag_sa_udt, {}, "help");
+ABSL_FLAG(NonTriviallyCopyableUDT, test_flag_ntc_udt, {}, "help");
+
+namespace {
+
+TEST_F(FlagTest, TestSmallAlignUDT) {
+  SmallAlignUDT value = absl::GetFlag(FLAGS_test_flag_sa_udt);
+  EXPECT_EQ(value.c, 'A');
+  EXPECT_EQ(value.s, 12);
+
+  value.c = 'B';
+  value.s = 45;
+  absl::SetFlag(&FLAGS_test_flag_sa_udt, value);
+  value = absl::GetFlag(FLAGS_test_flag_sa_udt);
+  EXPECT_EQ(value.c, 'B');
+  EXPECT_EQ(value.s, 45);
+}
+
+TEST_F(FlagTest, TestNonTriviallyCopyableUDT) {
+  NonTriviallyCopyableUDT value = absl::GetFlag(FLAGS_test_flag_ntc_udt);
+  EXPECT_EQ(value.c, 'A');
+
+  value.c = 'B';
+  absl::SetFlag(&FLAGS_test_flag_ntc_udt, value);
+  value = absl::GetFlag(FLAGS_test_flag_ntc_udt);
+  EXPECT_EQ(value.c, 'B');
+}
+
+}  // namespace

diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
index f3c424a..089567f 100644
--- a/absl/flags/internal/flag.cc
+++ b/absl/flags/internal/flag.cc

@@ -92,9 +92,9 @@
         counter_(counter) {}
 
   ~FlagState() override {
-    if (flag_impl_->ValueStorageKind() != FlagValueStorageKind::kHeapAllocated)
+    if (flag_impl_->ValueStorageKind() != FlagValueStorageKind::kAlignedBuffer)
       return;
-    flags_internal::Delete(flag_impl_->op_, value_.dynamic);
+    flags_internal::Delete(flag_impl_->op_, value_.heap_allocated);
   }
 
  private:
@@ -112,11 +112,11 @@
   // Flag and saved flag data.
   FlagImpl* flag_impl_;
   union SavedValue {
-    explicit SavedValue(void* v) : dynamic(v) {}
+    explicit SavedValue(void* v) : heap_allocated(v) {}
     explicit SavedValue(int64_t v) : one_word(v) {}
     explicit SavedValue(flags_internal::AlignedTwoWords v) : two_words(v) {}
 
-    void* dynamic;
+    void* heap_allocated;
     int64_t one_word;
     flags_internal::AlignedTwoWords two_words;
   } value_;
@@ -128,25 +128,33 @@
 ///////////////////////////////////////////////////////////////////////////////
 // Flag implementation, which does not depend on flag value type.
 
+DynValueDeleter::DynValueDeleter(FlagOpFn op_arg) : op(op_arg) {}
+
+void DynValueDeleter::operator()(void* ptr) const {
+  if (op == nullptr) return;
+
+  Delete(op, ptr);
+}
+
 void FlagImpl::Init() {
   new (&data_guard_) absl::Mutex;
 
   // At this point the default_value_ always points to gen_func.
-  std::unique_ptr<void, DynValueDeleter> init_value(
-      (*default_value_.gen_func)(), DynValueDeleter{op_});
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated:
-      HeapAllocatedValue() = init_value.release();
+    case FlagValueStorageKind::kAlignedBuffer:
+      (*default_value_.gen_func)(AlignedBufferValue());
       break;
     case FlagValueStorageKind::kOneWordAtomic: {
-      int64_t atomic_value;
-      std::memcpy(&atomic_value, init_value.get(), Sizeof(op_));
-      OneWordValue().store(atomic_value, std::memory_order_release);
+      alignas(int64_t) std::array<char, sizeof(int64_t)> buf{};
+      (*default_value_.gen_func)(buf.data());
+      auto value = absl::bit_cast<int64_t>(buf);
+      OneWordValue().store(value, std::memory_order_release);
       break;
     }
     case FlagValueStorageKind::kTwoWordsAtomic: {
-      AlignedTwoWords atomic_value{0, 0};
-      std::memcpy(&atomic_value, init_value.get(), Sizeof(op_));
+      alignas(AlignedTwoWords) std::array<char, sizeof(AlignedTwoWords)> buf{};
+      (*default_value_.gen_func)(buf.data());
+      auto atomic_value = absl::bit_cast<AlignedTwoWords>(buf);
       TwoWordsValue().store(atomic_value, std::memory_order_release);
       break;
     }
@@ -191,15 +199,16 @@
   if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
     res = flags_internal::Clone(op_, default_value_.dynamic_value);
   } else {
-    res = (*default_value_.gen_func)();
+    res = flags_internal::Alloc(op_);
+    (*default_value_.gen_func)(res);
   }
   return {res, DynValueDeleter{op_}};
 }
 
 void FlagImpl::StoreValue(const void* src) {
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated:
-      Copy(op_, src, HeapAllocatedValue());
+    case FlagValueStorageKind::kAlignedBuffer:
+      Copy(op_, src, AlignedBufferValue());
       break;
     case FlagValueStorageKind::kOneWordAtomic: {
       int64_t one_word_val = 0;
@@ -257,9 +266,9 @@
 std::string FlagImpl::CurrentValue() const {
   auto* guard = DataGuard();  // Make sure flag initialized
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated: {
+    case FlagValueStorageKind::kAlignedBuffer: {
       absl::MutexLock l(guard);
-      return flags_internal::Unparse(op_, HeapAllocatedValue());
+      return flags_internal::Unparse(op_, AlignedBufferValue());
     }
     case FlagValueStorageKind::kOneWordAtomic: {
       const auto one_word_val =
@@ -318,9 +327,9 @@
   bool modified = modified_;
   bool on_command_line = on_command_line_;
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated: {
+    case FlagValueStorageKind::kAlignedBuffer: {
       return absl::make_unique<FlagState>(
-          this, flags_internal::Clone(op_, HeapAllocatedValue()), modified,
+          this, flags_internal::Clone(op_, AlignedBufferValue()), modified,
           on_command_line, counter_);
     }
     case FlagValueStorageKind::kOneWordAtomic: {
@@ -345,8 +354,8 @@
   }
 
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated:
-      StoreValue(flag_state.value_.dynamic);
+    case FlagValueStorageKind::kAlignedBuffer:
+      StoreValue(flag_state.value_.heap_allocated);
       break;
     case FlagValueStorageKind::kOneWordAtomic:
       StoreValue(&flag_state.value_.one_word);
@@ -363,25 +372,27 @@
 }
 
 template <typename StorageT>
-typename StorageT::value_type& FlagImpl::OffsetValue() const {
+StorageT* FlagImpl::OffsetValue() const {
   char* p = reinterpret_cast<char*>(const_cast<FlagImpl*>(this));
   // The offset is deduced via Flag value type specific op_.
   size_t offset = flags_internal::ValueOffset(op_);
 
-  return reinterpret_cast<StorageT*>(p + offset)->value;
+  return reinterpret_cast<StorageT*>(p + offset);
 }
 
-void*& FlagImpl::HeapAllocatedValue() const {
-  assert(ValueStorageKind() == FlagValueStorageKind::kHeapAllocated);
-  return OffsetValue<FlagHeapAllocatedValue>();
+void* FlagImpl::AlignedBufferValue() const {
+  assert(ValueStorageKind() == FlagValueStorageKind::kAlignedBuffer);
+  return OffsetValue<void>();
 }
+
 std::atomic<int64_t>& FlagImpl::OneWordValue() const {
   assert(ValueStorageKind() == FlagValueStorageKind::kOneWordAtomic);
-  return OffsetValue<FlagOneWordValue>();
+  return OffsetValue<FlagOneWordValue>()->value;
 }
+
 std::atomic<AlignedTwoWords>& FlagImpl::TwoWordsValue() const {
   assert(ValueStorageKind() == FlagValueStorageKind::kTwoWordsAtomic);
-  return OffsetValue<FlagTwoWordsValue>();
+  return OffsetValue<FlagTwoWordsValue>()->value;
 }
 
 // Attempts to parse supplied `value` string using parsing routine in the `flag`
@@ -406,9 +417,9 @@
 void FlagImpl::Read(void* dst) const {
   auto* guard = DataGuard();  // Make sure flag initialized
   switch (ValueStorageKind()) {
-    case FlagValueStorageKind::kHeapAllocated: {
+    case FlagValueStorageKind::kAlignedBuffer: {
       absl::MutexLock l(guard);
-      flags_internal::CopyConstruct(op_, HeapAllocatedValue(), dst);
+      flags_internal::CopyConstruct(op_, AlignedBufferValue(), dst);
       break;
     }
     case FlagValueStorageKind::kOneWordAtomic: {

diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index c1bf865..6da25aa 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h

@@ -46,8 +46,8 @@
 // by function specific to that type with a signature matching FlagOpFn.
 
 enum class FlagOp {
+  kAlloc,
   kDelete,
-  kClone,
   kCopy,
   kCopyConstruct,
   kSizeof,
@@ -63,13 +63,13 @@
 template <typename T>
 void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3);
 
-// Deletes memory interpreting obj as flag value type pointer.
-inline void Delete(FlagOpFn op, const void* obj) {
-  op(FlagOp::kDelete, obj, nullptr, nullptr);
+// Allocate aligned memory for a flag value.
+inline void* Alloc(FlagOpFn op) {
+  return op(FlagOp::kAlloc, nullptr, nullptr, nullptr);
 }
-// Makes a copy of flag value pointed by obj.
-inline void* Clone(FlagOpFn op, const void* obj) {
-  return op(FlagOp::kClone, obj, nullptr, nullptr);
+// Deletes memory interpreting obj as flag value type pointer.
+inline void Delete(FlagOpFn op, void* obj) {
+  op(FlagOp::kDelete, nullptr, obj, nullptr);
 }
 // Copies src to dst interpreting as flag value type pointers.
 inline void Copy(FlagOpFn op, const void* src, void* dst) {
@@ -80,6 +80,12 @@
 inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
   op(FlagOp::kCopyConstruct, src, dst, nullptr);
 }
+// Makes a copy of flag value pointed by obj.
+inline void* Clone(FlagOpFn op, const void* obj) {
+  void* res = flags_internal::Alloc(op);
+  flags_internal::CopyConstruct(op, obj, res);
+  return res;
+}
 // Returns true if parsing of input text is successfull.
 inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
                   std::string* error) {
@@ -195,7 +201,7 @@
 
 // Signature for the function generating the initial flag value (usually
 // based on default value supplied in flag's definition)
-using FlagDfltGenFunc = void* (*)();
+using FlagDfltGenFunc = void (*)(void*);
 
 union FlagDefaultSrc {
   constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
@@ -253,46 +259,36 @@
 #endif
 
 template <typename T>
-using FlagUseHeapStorage =
+using FlagUseBufferStorage =
     std::integral_constant<bool, !FlagUseOneWordStorage<T>::value &&
                                      !FlagUseTwoWordsStorage<T>::value>;
 
 enum class FlagValueStorageKind : uint8_t {
-  kHeapAllocated = 0,
+  kAlignedBuffer = 0,
   kOneWordAtomic = 1,
   kTwoWordsAtomic = 2
 };
 
 template <typename T>
 static constexpr FlagValueStorageKind StorageKind() {
-  return FlagUseHeapStorage<T>::value
-             ? FlagValueStorageKind::kHeapAllocated
+  return FlagUseBufferStorage<T>::value
+             ? FlagValueStorageKind::kAlignedBuffer
              : FlagUseOneWordStorage<T>::value
                    ? FlagValueStorageKind::kOneWordAtomic
-                   : FlagUseTwoWordsStorage<T>::value
-                         ? FlagValueStorageKind::kTwoWordsAtomic
-                         : FlagValueStorageKind::kHeapAllocated;
+                   : FlagValueStorageKind::kTwoWordsAtomic;
 }
 
-struct FlagHeapAllocatedValue {
-  using value_type = void*;
-
-  value_type value;
-};
-
 struct FlagOneWordValue {
-  using value_type = std::atomic<int64_t>;
   constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {}
 
-  value_type value;
+  std::atomic<int64_t> value;
 };
 
 struct FlagTwoWordsValue {
-  using value_type = std::atomic<AlignedTwoWords>;
   constexpr FlagTwoWordsValue()
       : value(AlignedTwoWords{UninitializedFlagValue(), 0}) {}
 
-  value_type value;
+  std::atomic<AlignedTwoWords> value;
 };
 
 template <typename T,
@@ -300,9 +296,10 @@
 struct FlagValue;
 
 template <typename T>
-struct FlagValue<T, FlagValueStorageKind::kHeapAllocated>
-    : FlagHeapAllocatedValue {
+struct FlagValue<T, FlagValueStorageKind::kAlignedBuffer> {
   bool Get(T*) const { return false; }
+
+  alignas(T) char value[sizeof(T)];
 };
 
 template <typename T>
@@ -347,10 +344,8 @@
 // The class encapsulates the Flag's data and access to it.
 
 struct DynValueDeleter {
-  explicit DynValueDeleter(FlagOpFn op_arg = nullptr) : op(op_arg) {}
-  void operator()(void* ptr) const {
-    if (op != nullptr) flags_internal::Delete(op, ptr);
-  }
+  explicit DynValueDeleter(FlagOpFn op_arg = nullptr);
+  void operator()(void* ptr) const;
 
   FlagOpFn op;
 };
@@ -416,10 +411,10 @@
   // it is only used inside the three routines below, which are defined in
   // flag.cc, we can define it in that file as well.
   template <typename StorageT>
-  typename StorageT::value_type& OffsetValue() const;
-  // This is an accessor for a value stored in heap allocated storage.
-  // Returns a mutable reference to a pointer to allow vlaue mutation.
-  void*& HeapAllocatedValue() const;
+  StorageT* OffsetValue() const;
+  // This is an accessor for a value stored in an aligned buffer storage.
+  // Returns a mutable pointer to the start of a buffer.
+  void* AlignedBufferValue() const;
   // This is an accessor for a value stored as one word atomic. Returns a
   // mutable reference to an atomic value.
   std::atomic<int64_t>& OneWordValue() const;
@@ -492,17 +487,8 @@
   // Kind of storage this flag is using for the flag's value.
   const uint8_t value_storage_kind_ : 2;
 
-  // ------------------------------------------------------------------------
-  // The bytes containing the const bitfields must not be shared with bytes
-  // containing the mutable bitfields.
-  // ------------------------------------------------------------------------
-
-  // Unique tag for absl::call_once call to initialize this flag.
-  //
-  // The placement of this variable between the immutable and mutable bitfields
-  // is important as prevents them from occupying the same byte. If you remove
-  // this variable, make sure to maintain this property.
-  absl::once_flag init_control_;
+  uint8_t : 0;  // The bytes containing the const bitfields must not be
+                // shared with bytes containing the mutable bitfields.
 
   // Mutable flag's state (guarded by `data_guard_`).
 
@@ -514,6 +500,9 @@
   // Has this flag been specified on command line.
   bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());
 
+  // Unique tag for absl::call_once call to initialize this flag.
+  absl::once_flag init_control_;
+
   // Mutation counter
   int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
   // Optional flag's callback and absl::Mutex to guard the invocations.
@@ -567,9 +556,6 @@
     impl_.AssertValidType(base_internal::FastTypeId<T>(), &GenRuntimeTypeId<T>);
     impl_.Write(&v);
   }
-  void SetCallback(const FlagCallbackFunc mutation_callback) {
-    impl_.SetCallback(mutation_callback);
-  }
 
   // CommandLineFlag interface
   absl::string_view Name() const { return impl_.Name(); }
@@ -600,11 +586,17 @@
 template <typename T>
 void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
   switch (op) {
-    case FlagOp::kDelete:
-      delete static_cast<const T*>(v1);
+    case FlagOp::kAlloc: {
+      std::allocator<T> alloc;
+      return std::allocator_traits<std::allocator<T>>::allocate(alloc, 1);
+    }
+    case FlagOp::kDelete: {
+      T* p = static_cast<T*>(v2);
+      p->~T();
+      std::allocator<T> alloc;
+      std::allocator_traits<std::allocator<T>>::deallocate(alloc, p, 1);
       return nullptr;
-    case FlagOp::kClone:
-      return new T(*static_cast<const T*>(v1));
+    }
     case FlagOp::kCopy:
       *static_cast<T*>(v2) = *static_cast<const T*>(v1);
       return nullptr;
@@ -648,6 +640,7 @@
 // This class facilitates Flag object registration and tail expression-based
 // flag definition, for example:
 // ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
+struct FlagRegistrarEmpty {};
 template <typename T, bool do_register>
 class FlagRegistrar {
  public:
@@ -655,14 +648,15 @@
     if (do_register) flags_internal::RegisterCommandLineFlag(&flag_->impl_);
   }
 
-  FlagRegistrar& OnUpdate(FlagCallbackFunc cb) && {
-    flag_->SetCallback(cb);
+  FlagRegistrar OnUpdate(FlagCallbackFunc cb) && {
+    flag_->impl_.SetCallback(cb);
     return *this;
   }
 
-  // Make the registrar "die" gracefully as a bool on a line where registration
-  // happens. Registrar objects are intended to live only as temporary.
-  operator bool() const { return true; }  // NOLINT
+  // Make the registrar "die" gracefully as an empty struct on a line where
+  // registration happens. Registrar objects are intended to live only as
+  // temporary.
+  operator FlagRegistrarEmpty() const { return {}; }  // NOLINT
 
  private:
   Flag<T>* flag_;  // Flag being registered (not owned).
@@ -673,13 +667,13 @@
 struct EmptyBraces {};
 
 template <typename T>
-T* MakeFromDefaultValue(T t) {
-  return new T(std::move(t));
+void MakeFromDefaultValue(void* dst, T t) {
+  new (dst) T(std::move(t));
 }
 
 template <typename T>
-T* MakeFromDefaultValue(EmptyBraces) {
-  return new T{};
+void MakeFromDefaultValue(void* dst, EmptyBraces) {
+  new (dst) T{};
 }
 
 }  // namespace flags_internal

diff --git a/absl/flags/internal/usage_test.cc b/absl/flags/internal/usage_test.cc
index e1e57e5..8dd3532 100644
--- a/absl/flags/internal/usage_test.cc
+++ b/absl/flags/internal/usage_test.cc

@@ -103,8 +103,9 @@
 
 #ifndef _WIN32
   // TODO(rogeeff): figure out why this does not work on Windows.
-  EXPECT_DEATH(absl::SetProgramUsageMessage("custom usage message"),
-               ".*SetProgramUsageMessage\\(\\) called twice.*");
+  EXPECT_DEATH_IF_SUPPORTED(
+      absl::SetProgramUsageMessage("custom usage message"),
+      ".*SetProgramUsageMessage\\(\\) called twice.*");
 #endif
 }
 

diff --git a/absl/flags/parse_test.cc b/absl/flags/parse_test.cc
index 6f49377..065f757 100644
--- a/absl/flags/parse_test.cc
+++ b/absl/flags/parse_test.cc

@@ -481,21 +481,22 @@
       "testbin",
       "--undefined_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
-               "Unknown command line flag 'undefined_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
+                            "Unknown command line flag 'undefined_flag'");
 
   const char* in_args2[] = {
       "testbin",
       "--noprefixed_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
-               "Unknown command line flag 'noprefixed_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
+                            "Unknown command line flag 'noprefixed_flag'");
 
   const char* in_args3[] = {
       "testbin",
       "--Int_flag=1",
   };
-  EXPECT_DEATH(InvokeParse(in_args3), "Unknown command line flag 'Int_flag'");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args3),
+                            "Unknown command line flag 'Int_flag'");
 }
 
 // --------------------------------------------------------------------
@@ -505,7 +506,7 @@
       "testbin",
       "--bool_flag=",
   };
-  EXPECT_DEATH(
+  EXPECT_DEATH_IF_SUPPORTED(
       InvokeParse(in_args1),
       "Missing the value after assignment for the boolean flag 'bool_flag'");
 
@@ -513,7 +514,7 @@
       "testbin",
       "--nobool_flag=true",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Negative form with assignment is not valid for the boolean "
                "flag 'bool_flag'");
 }
@@ -525,14 +526,14 @@
       "testbin",
       "--nostring_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Negative form is not valid for the flag 'string_flag'");
 
   const char* in_args2[] = {
       "testbin",
       "--int_flag",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Missing the value for the flag 'int_flag'");
 }
 
@@ -543,7 +544,7 @@
       "testbin",
       "--udt_flag=1",
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Illegal value '1' specified for flag 'udt_flag'; Use values A, "
                "AAA instead");
 
@@ -552,7 +553,7 @@
       "--udt_flag",
       "AA",
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Illegal value 'AA' specified for flag 'udt_flag'; Use values "
                "A, AAA instead");
 }
@@ -658,7 +659,7 @@
       GetFlagfileFlag({{"parse_test.ff4",
                         absl::MakeConstSpan(ff4_data)}}, &flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "Unknown command line flag 'unknown_flag'");
 
   constexpr const char* const ff5_data[] = {
@@ -670,7 +671,7 @@
       GetFlagfileFlag({{"parse_test.ff5",
                         absl::MakeConstSpan(ff5_data)}}, &flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args2),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args2),
                "Unknown command line flag 'int_flag 10'");
 
   constexpr const char* const ff6_data[] = {
@@ -682,14 +683,15 @@
       GetFlagfileFlag({{"parse_test.ff6", absl::MakeConstSpan(ff6_data)}},
                       &flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args3),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args3),
                "Flagfile can't contain position arguments or --");
 
   const char* in_args4[] = {
       "testbin",
       "--flagfile=invalid_flag_file",
   };
-  EXPECT_DEATH(InvokeParse(in_args4), "Can't open flagfile invalid_flag_file");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args4),
+                            "Can't open flagfile invalid_flag_file");
 
   constexpr const char* const ff7_data[] = {
       "--int_flag=10",
@@ -702,7 +704,7 @@
       GetFlagfileFlag({{"parse_test.ff7", absl::MakeConstSpan(ff7_data)}},
                       &flagfile_flag),
   };
-  EXPECT_DEATH(InvokeParse(in_args5),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args5),
                "Unexpected line in the flagfile .*: \\*bin\\*");
 }
 
@@ -724,7 +726,7 @@
 TEST_F(ParseDeathTest, TestReadingUnsetRequiredFlagsFromEnv) {
   const char* in_args1[] = {"testbin", "--fromenv=int_flag"};
 
-  EXPECT_DEATH(InvokeParse(in_args1),
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
                "FLAGS_int_flag not found in environment");
 }
 
@@ -735,7 +737,8 @@
 
   ScopedSetEnv set_tryfromenv("FLAGS_tryfromenv", "int_flag");
 
-  EXPECT_DEATH(InvokeParse(in_args1), "Infinite recursion on flag tryfromenv");
+  EXPECT_DEATH_IF_SUPPORTED(InvokeParse(in_args1),
+                            "Infinite recursion on flag tryfromenv");
 }
 
 // --------------------------------------------------------------------

diff --git a/absl/hash/BUILD.bazel b/absl/hash/BUILD.bazel
index 59eac78..6c77f1a 100644
--- a/absl/hash/BUILD.bazel
+++ b/absl/hash/BUILD.bazel

@@ -43,7 +43,6 @@
         "//absl/meta:type_traits",
         "//absl/numeric:int128",
         "//absl/strings",
-        "//absl/strings:cord",
         "//absl/types:optional",
         "//absl/types:variant",
         "//absl/utility",

diff --git a/absl/hash/CMakeLists.txt b/absl/hash/CMakeLists.txt
index 4e55514..61365e9 100644
--- a/absl/hash/CMakeLists.txt
+++ b/absl/hash/CMakeLists.txt

@@ -25,7 +25,6 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::cord
     absl::core_headers
     absl::endian
     absl::fixed_array

diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 025d287..a71bd4a 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h

@@ -43,7 +43,6 @@
 #include "absl/container/fixed_array.h"
 #include "absl/meta/type_traits.h"
 #include "absl/numeric/int128.h"
-#include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
@@ -54,12 +53,65 @@
 ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
-class PiecewiseCombiner;
-
 // Internal detail: Large buffers are hashed in smaller chunks.  This function
 // returns the size of these chunks.
 constexpr size_t PiecewiseChunkSize() { return 1024; }
 
+// PiecewiseCombiner
+//
+// PiecewiseCombiner is an internal-only helper class for hashing a piecewise
+// buffer of `char` or `unsigned char` as though it were contiguous.  This class
+// provides two methods:
+//
+//   H add_buffer(state, data, size)
+//   H finalize(state)
+//
+// `add_buffer` can be called zero or more times, followed by a single call to
+// `finalize`.  This will produce the same hash expansion as concatenating each
+// buffer piece into a single contiguous buffer, and passing this to
+// `H::combine_contiguous`.
+//
+//  Example usage:
+//    PiecewiseCombiner combiner;
+//    for (const auto& piece : pieces) {
+//      state = combiner.add_buffer(std::move(state), piece.data, piece.size);
+//    }
+//    return combiner.finalize(std::move(state));
+class PiecewiseCombiner {
+ public:
+  PiecewiseCombiner() : position_(0) {}
+  PiecewiseCombiner(const PiecewiseCombiner&) = delete;
+  PiecewiseCombiner& operator=(const PiecewiseCombiner&) = delete;
+
+  // PiecewiseCombiner::add_buffer()
+  //
+  // Appends the given range of bytes to the sequence to be hashed, which may
+  // modify the provided hash state.
+  template <typename H>
+  H add_buffer(H state, const unsigned char* data, size_t size);
+  template <typename H>
+  H add_buffer(H state, const char* data, size_t size) {
+    return add_buffer(std::move(state),
+                      reinterpret_cast<const unsigned char*>(data), size);
+  }
+
+  // PiecewiseCombiner::finalize()
+  //
+  // Finishes combining the hash sequence, which may may modify the provided
+  // hash state.
+  //
+  // Once finalize() is called, add_buffer() may no longer be called. The
+  // resulting hash state will be the same as if the pieces passed to
+  // add_buffer() were concatenated into a single flat buffer, and then provided
+  // to H::combine_contiguous().
+  template <typename H>
+  H finalize(H state);
+
+ private:
+  unsigned char buf_[PiecewiseChunkSize()];
+  size_t position_;
+};
+
 // HashStateBase
 //
 // A hash state object represents an intermediate state in the computation
@@ -126,8 +178,7 @@
   template <typename T>
   static H combine_contiguous(H state, const T* data, size_t size);
 
- private:
-  friend class PiecewiseCombiner;
+  using AbslInternalPiecewiseCombiner = PiecewiseCombiner;
 };
 
 // is_uniquely_represented
@@ -198,61 +249,6 @@
   return H::combine_contiguous(std::move(hash_state), start, sizeof(value));
 }
 
-// PiecewiseCombiner
-//
-// PiecewiseCombiner is an internal-only helper class for hashing a piecewise
-// buffer of `char` or `unsigned char` as though it were contiguous.  This class
-// provides two methods:
-//
-//   H add_buffer(state, data, size)
-//   H finalize(state)
-//
-// `add_buffer` can be called zero or more times, followed by a single call to
-// `finalize`.  This will produce the same hash expansion as concatenating each
-// buffer piece into a single contiguous buffer, and passing this to
-// `H::combine_contiguous`.
-//
-//  Example usage:
-//    PiecewiseCombiner combiner;
-//    for (const auto& piece : pieces) {
-//      state = combiner.add_buffer(std::move(state), piece.data, piece.size);
-//    }
-//    return combiner.finalize(std::move(state));
-class PiecewiseCombiner {
- public:
-  PiecewiseCombiner() : position_(0) {}
-  PiecewiseCombiner(const PiecewiseCombiner&) = delete;
-  PiecewiseCombiner& operator=(const PiecewiseCombiner&) = delete;
-
-  // PiecewiseCombiner::add_buffer()
-  //
-  // Appends the given range of bytes to the sequence to be hashed, which may
-  // modify the provided hash state.
-  template <typename H>
-  H add_buffer(H state, const unsigned char* data, size_t size);
-  template <typename H>
-  H add_buffer(H state, const char* data, size_t size) {
-    return add_buffer(std::move(state),
-                      reinterpret_cast<const unsigned char*>(data), size);
-  }
-
-  // PiecewiseCombiner::finalize()
-  //
-  // Finishes combining the hash sequence, which may may modify the provided
-  // hash state.
-  //
-  // Once finalize() is called, add_buffer() may no longer be called. The
-  // resulting hash state will be the same as if the pieces passed to
-  // add_buffer() were concatenated into a single flat buffer, and then provided
-  // to H::combine_contiguous().
-  template <typename H>
-  H finalize(H state);
-
- private:
-  unsigned char buf_[PiecewiseChunkSize()];
-  size_t position_;
-};
-
 // -----------------------------------------------------------------------------
 // AbslHashValue for Basic Types
 // -----------------------------------------------------------------------------
@@ -443,25 +439,6 @@
       str.size());
 }
 
-template <typename H>
-H HashFragmentedCord(H hash_state, const absl::Cord& c) {
-  PiecewiseCombiner combiner;
-  c.ForEachChunk([&combiner, &hash_state](absl::string_view chunk) {
-    hash_state =
-        combiner.add_buffer(std::move(hash_state), chunk.data(), chunk.size());
-  });
-  return H::combine(combiner.finalize(std::move(hash_state)), c.size());
-}
-
-template <typename H>
-H AbslHashValue(H hash_state, const absl::Cord& c) {
-  absl::optional<absl::string_view> maybe_flat = c.TryFlat();
-  if (maybe_flat.has_value()) {
-    return H::combine(std::move(hash_state), *maybe_flat);
-  }
-  return hash_internal::HashFragmentedCord(std::move(hash_state), c);
-}
-
 // -----------------------------------------------------------------------------
 // AbslHashValue for Sequence Containers
 // -----------------------------------------------------------------------------

diff --git a/absl/random/BUILD.bazel b/absl/random/BUILD.bazel
index 4d94e1b..e61d31b 100644
--- a/absl/random/BUILD.bazel
+++ b/absl/random/BUILD.bazel

@@ -412,6 +412,7 @@
     name = "mocking_bit_gen_test",
     size = "small",
     srcs = ["mocking_bit_gen_test.cc"],
+    copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":bit_gen_ref",
@@ -426,6 +427,8 @@
     name = "mock_distributions_test",
     size = "small",
     srcs = ["mock_distributions_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":mock_distributions",
         ":mocking_bit_gen",

diff --git a/absl/random/internal/iostream_state_saver.h b/absl/random/internal/iostream_state_saver.h
index 7378829..e6e242e 100644
--- a/absl/random/internal/iostream_state_saver.h
+++ b/absl/random/internal/iostream_state_saver.h

@@ -192,8 +192,8 @@
 
   template <typename OStream>
   inline void write(absl::uint128 val, OStream& out) {
-    uint64_t h = Uint128High64(val);
-    uint64_t l = Uint128Low64(val);
+    uint64_t h = absl::Uint128High64(val);
+    uint64_t l = absl::Uint128Low64(val);
     out << h << out.fill() << l;
   }
 };

diff --git a/absl/random/internal/nanobenchmark_test.cc b/absl/random/internal/nanobenchmark_test.cc
index ab824ef..f1571e2 100644
--- a/absl/random/internal/nanobenchmark_test.cc
+++ b/absl/random/internal/nanobenchmark_test.cc

@@ -53,7 +53,7 @@
   // Avoid migrating between cores - important on multi-socket systems.
   int cpu = -1;
   if (argc == 2) {
-    if (!SimpleAtoi(argv[1], &cpu)) {
+    if (!absl::SimpleAtoi(argv[1], &cpu)) {
       ABSL_RAW_LOG(FATAL, "The optional argument must be a CPU number >= 0.\n");
     }
   }

diff --git a/absl/random/internal/wide_multiply.h b/absl/random/internal/wide_multiply.h
index 6e4cf1b..0afcbe0 100644
--- a/absl/random/internal/wide_multiply.h
+++ b/absl/random/internal/wide_multiply.h

@@ -38,9 +38,9 @@
 // MultiplyU64ToU128 multiplies two 64-bit values to a 128-bit value.
 // If an intrinsic is available, it is used, otherwise use native 32-bit
 // multiplies to construct the result.
-inline uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
+inline absl::uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
 #if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return uint128(static_cast<__uint128_t>(a) * b);
+  return absl::uint128(static_cast<__uint128_t>(a) * b);
 #elif defined(ABSL_INTERNAL_USE_UMUL128)
   // uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC.
   uint64_t high = 0;
@@ -93,14 +93,14 @@
 template <>
 struct wide_multiply<uint64_t> {
   using input_type = uint64_t;
-  using result_type = uint128;
+  using result_type = absl::uint128;
 
   static result_type multiply(uint64_t a, uint64_t b) {
     return MultiplyU64ToU128(a, b);
   }
 
-  static uint64_t hi(result_type r) { return Uint128High64(r); }
-  static uint64_t lo(result_type r) { return Uint128Low64(r); }
+  static uint64_t hi(result_type r) { return absl::Uint128High64(r); }
+  static uint64_t lo(result_type r) { return absl::Uint128Low64(r); }
 };
 #endif
 

diff --git a/absl/random/random.h b/absl/random/random.h
index c8f326e..71b6309 100644
--- a/absl/random/random.h
+++ b/absl/random/random.h

@@ -109,7 +109,7 @@
 
 // absl::BitGen::max()
 //
-// Returns the largest possible value from this bit generator., and
+// Returns the largest possible value from this bit generator.
 
 // absl::BitGen::discard(num)
 //

diff --git a/absl/status/BUILD.bazel b/absl/status/BUILD.bazel
index 2b83077..d164252 100644
--- a/absl/status/BUILD.bazel
+++ b/absl/status/BUILD.bazel

@@ -40,6 +40,7 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     deps = [
+        "//absl/base:atomic_hook",
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",

diff --git a/absl/status/CMakeLists.txt b/absl/status/CMakeLists.txt
index f05cee5..c041d69 100644
--- a/absl/status/CMakeLists.txt
+++ b/absl/status/CMakeLists.txt

@@ -25,6 +25,7 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::atomic_hook
     absl::config
     absl::core_headers
     absl::raw_logging_internal

diff --git a/absl/status/status.h b/absl/status/status.h
index 67ff988..967e606 100644
--- a/absl/status/status.h
+++ b/absl/status/status.h

@@ -78,7 +78,7 @@
   Status();
 
   // Create a status in the canonical error space with the specified code and
-  // error message.  If `code == util::error::OK`, `msg` is ignored and an
+  // error message.  If `code == absl::StatusCode::kOk`, `msg` is ignored and an
   // object identical to an OK status is constructed.
   //
   // `msg` must be in UTF-8. The implementation may complain (e.g.,

diff --git a/absl/status/status_payload_printer.cc b/absl/status/status_payload_printer.cc
index ad96d76..a47aea1 100644
--- a/absl/status/status_payload_printer.cc
+++ b/absl/status/status_payload_printer.cc

@@ -16,26 +16,21 @@
 #include <atomic>
 
 #include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace status_internal {
 
-namespace {
-// Tried constant initialized global variable but it doesn't work with Lexan
-// (MSVC's `std::atomic` has trouble constant initializing).
-std::atomic<StatusPayloadPrinter>& GetStatusPayloadPrinterStorage() {
-  ABSL_CONST_INIT static std::atomic<StatusPayloadPrinter> instance{nullptr};
-  return instance;
-}
-}  // namespace
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+static absl::base_internal::AtomicHook<StatusPayloadPrinter> storage;
 
 void SetStatusPayloadPrinter(StatusPayloadPrinter printer) {
-  GetStatusPayloadPrinterStorage().store(printer, std::memory_order_relaxed);
+  storage.Store(printer);
 }
 
 StatusPayloadPrinter GetStatusPayloadPrinter() {
-  return GetStatusPayloadPrinterStorage().load(std::memory_order_relaxed);
+  return storage.Load();
 }
 
 }  // namespace status_internal

diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 3890112..8aecbe5 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel

@@ -310,6 +310,7 @@
     deps = [
         ":cord",
         ":cord_test_helpers",
+        ":str_format",
         ":strings",
         "//absl/base",
         "//absl/base:config",
@@ -368,6 +369,8 @@
         ":strings",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
+        "//absl/container:flat_hash_map",
+        "//absl/container:node_hash_map",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -649,6 +652,7 @@
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
         ":str_format",
         ":strings",
         "//absl/base:core_headers",
@@ -666,6 +670,7 @@
     deps = [
         ":str_format",
         ":str_format_internal",
+        ":strings",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -724,6 +729,7 @@
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
         ":str_format_internal",
         "@com_google_googletest//:gtest_main",
     ],

diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index d3a8bd7..003794f 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt

@@ -210,6 +210,8 @@
     absl::base
     absl::core_headers
     absl::dynamic_annotations
+    absl::flat_hash_map
+    absl::node_hash_map
     gmock_main
 )
 
@@ -407,6 +409,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format
+    absl::cord
     absl::strings
     absl::core_headers
     gmock_main
@@ -422,6 +425,7 @@
   DEPS
     absl::str_format
     absl::str_format_internal
+    absl::strings
     gmock_main
 )
 
@@ -485,6 +489,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format_internal
+    absl::cord
     gmock_main
 )
 
@@ -539,7 +544,6 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::strings_internal
     absl::base
     absl::base_internal
     absl::core_headers
@@ -550,6 +554,7 @@
     absl::optional
     absl::raw_logging_internal
     absl::strings
+    absl::strings_internal
     absl::type_traits
   PUBLIC
 )
@@ -575,6 +580,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::cord
+    absl::str_format
     absl::strings
     absl::base
     absl::config

diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
index 7de7766..fa490c1 100644
--- a/absl/strings/cord.cc
+++ b/absl/strings/cord.cc

@@ -31,6 +31,7 @@
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 #include "absl/container/fixed_array.h"
+#include "absl/container/inlined_vector.h"
 #include "absl/strings/escaping.h"
 #include "absl/strings/internal/cord_internal.h"
 #include "absl/strings/internal/resize_uninitialized.h"
@@ -132,14 +133,6 @@
   return static_cast<const CordRepExternal*>(this);
 }
 
-using CordTreeConstPath = CordTreePath<const CordRep*, MaxCordDepth()>;
-
-// This type is used to store the list of pending nodes during re-balancing.
-// Its maximum size is 2 * MaxCordDepth() because the tree has a maximum
-// possible depth of MaxCordDepth() and every concat node along a tree path
-// could theoretically be split during rebalancing.
-using RebalancingStack = CordTreePath<CordRep*, 2 * MaxCordDepth()>;
-
 }  // namespace cord_internal
 
 static const size_t kFlatOverhead = offsetof(CordRep, data);
@@ -188,78 +181,64 @@
 // Enforce that kMaxFlatSize maps to a well-known exact tag value.
 static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic");
 
-constexpr size_t Fibonacci(uint8_t n, const size_t a = 0, const size_t b = 1) {
-  return n == 0
-             ? a
-             : n == 1 ? b
-                      : Fibonacci(n - 1, b,
-                                  (a > (size_t(-1) - b)) ? size_t(-1) : a + b);
+constexpr uint64_t Fibonacci(unsigned char n, uint64_t a = 0, uint64_t b = 1) {
+  return n == 0 ? a : Fibonacci(n - 1, b, a + b);
 }
 
+static_assert(Fibonacci(63) == 6557470319842,
+              "Fibonacci values computed incorrectly");
+
 // Minimum length required for a given depth tree -- a tree is considered
 // balanced if
-//      length(t) >= kMinLength[depth(t)]
-// The node depth is allowed to become larger to reduce rebalancing
-// for larger strings (see ShouldRebalance).
-constexpr size_t kMinLength[] = {
-    Fibonacci(2),  Fibonacci(3),  Fibonacci(4),  Fibonacci(5),  Fibonacci(6),
-    Fibonacci(7),  Fibonacci(8),  Fibonacci(9),  Fibonacci(10), Fibonacci(11),
-    Fibonacci(12), Fibonacci(13), Fibonacci(14), Fibonacci(15), Fibonacci(16),
-    Fibonacci(17), Fibonacci(18), Fibonacci(19), Fibonacci(20), Fibonacci(21),
-    Fibonacci(22), Fibonacci(23), Fibonacci(24), Fibonacci(25), Fibonacci(26),
-    Fibonacci(27), Fibonacci(28), Fibonacci(29), Fibonacci(30), Fibonacci(31),
-    Fibonacci(32), Fibonacci(33), Fibonacci(34), Fibonacci(35), Fibonacci(36),
-    Fibonacci(37), Fibonacci(38), Fibonacci(39), Fibonacci(40), Fibonacci(41),
-    Fibonacci(42), Fibonacci(43), Fibonacci(44), Fibonacci(45), Fibonacci(46),
-    Fibonacci(47), Fibonacci(48), Fibonacci(49), Fibonacci(50), Fibonacci(51),
-    Fibonacci(52), Fibonacci(53), Fibonacci(54), Fibonacci(55), Fibonacci(56),
-    Fibonacci(57), Fibonacci(58), Fibonacci(59), Fibonacci(60), Fibonacci(61),
-    Fibonacci(62), Fibonacci(63), Fibonacci(64), Fibonacci(65), Fibonacci(66),
-    Fibonacci(67), Fibonacci(68), Fibonacci(69), Fibonacci(70), Fibonacci(71),
-    Fibonacci(72), Fibonacci(73), Fibonacci(74), Fibonacci(75), Fibonacci(76),
-    Fibonacci(77), Fibonacci(78), Fibonacci(79), Fibonacci(80), Fibonacci(81),
-    Fibonacci(82), Fibonacci(83), Fibonacci(84), Fibonacci(85), Fibonacci(86),
-    Fibonacci(87), Fibonacci(88), Fibonacci(89), Fibonacci(90), Fibonacci(91),
-    Fibonacci(92), Fibonacci(93), Fibonacci(94), Fibonacci(95)};
+//      length(t) >= min_length[depth(t)]
+// The root node depth is allowed to become twice as large to reduce rebalancing
+// for larger strings (see IsRootBalanced).
+static constexpr uint64_t min_length[] = {
+    Fibonacci(2),          Fibonacci(3),  Fibonacci(4),  Fibonacci(5),
+    Fibonacci(6),          Fibonacci(7),  Fibonacci(8),  Fibonacci(9),
+    Fibonacci(10),         Fibonacci(11), Fibonacci(12), Fibonacci(13),
+    Fibonacci(14),         Fibonacci(15), Fibonacci(16), Fibonacci(17),
+    Fibonacci(18),         Fibonacci(19), Fibonacci(20), Fibonacci(21),
+    Fibonacci(22),         Fibonacci(23), Fibonacci(24), Fibonacci(25),
+    Fibonacci(26),         Fibonacci(27), Fibonacci(28), Fibonacci(29),
+    Fibonacci(30),         Fibonacci(31), Fibonacci(32), Fibonacci(33),
+    Fibonacci(34),         Fibonacci(35), Fibonacci(36), Fibonacci(37),
+    Fibonacci(38),         Fibonacci(39), Fibonacci(40), Fibonacci(41),
+    Fibonacci(42),         Fibonacci(43), Fibonacci(44), Fibonacci(45),
+    Fibonacci(46),         Fibonacci(47),
+    0xffffffffffffffffull,  // Avoid overflow
+};
 
-static_assert(sizeof(kMinLength) / sizeof(size_t) >=
-                  (cord_internal::MaxCordDepth() + 1),
-              "Not enough elements in kMinLength array to cover all the "
-              "supported Cord depth(s)");
+static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
 
-inline bool ShouldRebalance(const CordRep* node) {
-  if (node->tag != CONCAT) return false;
+// The inlined size to use with absl::InlinedVector.
+//
+// Note: The InlinedVectors in this file (and in cord.h) do not need to use
+// the same value for their inlined size. The fact that they do is historical.
+// It may be desirable for each to use a different inlined size optimized for
+// that InlinedVector's usage.
+//
+// TODO(jgm): Benchmark to see if there's a more optimal value than 47 for
+// the inlined vector size (47 exists for backward compatibility).
+static const int kInlinedVectorSize = 47;
 
-  size_t node_depth = node->concat()->depth();
-
-  if (node_depth <= 15) return false;
-
-  // Rebalancing Cords is expensive, so we reduce how often rebalancing occurs
-  // by allowing shallow Cords to have twice the depth that the Fibonacci rule
-  // would otherwise imply. Deep Cords need to follow the rule more closely,
-  // however to ensure algorithm correctness. We implement this with linear
-  // interpolation. Cords of depth 16 are treated as though they have a depth
-  // of 16 * 1/2, and Cords of depth MaxCordDepth() interpolate to
-  // MaxCordDepth() * 1.
-  return node->length <
-         kMinLength[(node_depth * (cord_internal::MaxCordDepth() - 16)) /
-                    (2 * cord_internal::MaxCordDepth() - 16 - node_depth)];
-}
-
-// Unlike root balancing condition this one is part of the re-balancing
-// algorithm and has to be always matching against right depth for
-// algorithm to be correct.
-inline bool IsNodeBalanced(const CordRep* node) {
-  if (node->tag != CONCAT) return true;
-
-  size_t node_depth = node->concat()->depth();
-
-  return node->length >= kMinLength[node_depth];
+static inline bool IsRootBalanced(CordRep* node) {
+  if (node->tag != CONCAT) {
+    return true;
+  } else if (node->concat()->depth() <= 15) {
+    return true;
+  } else if (node->concat()->depth() > kMinLengthSize) {
+    return false;
+  } else {
+    // Allow depth to become twice as large as implied by fibonacci rule to
+    // reduce rebalancing for larger strings.
+    return (node->length >= min_length[node->concat()->depth() / 2]);
+  }
 }
 
 static CordRep* Rebalance(CordRep* node);
-static void DumpNode(const CordRep* rep, bool include_data, std::ostream* os);
-static bool VerifyNode(const CordRep* root, const CordRep* start_node,
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os);
+static bool VerifyNode(CordRep* root, CordRep* start_node,
                        bool full_validation);
 
 static inline CordRep* VerifyTree(CordRep* node) {
@@ -306,8 +285,7 @@
 static void UnrefInternal(CordRep* rep) {
   assert(rep != nullptr);
 
-  cord_internal::RebalancingStack pending;
-
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> pending;
   while (true) {
     if (rep->tag == CONCAT) {
       CordRepConcat* rep_concat = rep->concat();
@@ -389,11 +367,6 @@
 
   concat->length = left->length + right->length;
   concat->set_depth(1 + std::max(Depth(left), Depth(right)));
-
-  ABSL_INTERNAL_CHECK(concat->depth() <= cord_internal::MaxCordDepth(),
-                      "Cord depth exceeds max");
-  ABSL_INTERNAL_CHECK(concat->length >= left->length, "Cord is too long");
-  ABSL_INTERNAL_CHECK(concat->length >= right->length, "Cord is too long");
 }
 
 // Create a concatenation of the specified nodes.
@@ -419,7 +392,7 @@
 
 static CordRep* Concat(CordRep* left, CordRep* right) {
   CordRep* rep = RawConcat(left, right);
-  if (rep != nullptr && ShouldRebalance(rep)) {
+  if (rep != nullptr && !IsRootBalanced(rep)) {
     rep = Rebalance(rep);
   }
   return VerifyTree(rep);
@@ -714,14 +687,6 @@
   memset(data_, 0, sizeof(data_));
 }
 
-inline Cord::InternalChunkIterator Cord::internal_chunk_begin() const {
-  return InternalChunkIterator(this);
-}
-
-inline Cord::InternalChunkRange Cord::InternalChunks() const {
-  return InternalChunkRange(this);
-}
-
 // --------------------------------------------------------------------
 // Constructors and destructors
 
@@ -918,7 +883,7 @@
 static CordRep* RemovePrefixFrom(CordRep* node, size_t n) {
   if (n >= node->length) return nullptr;
   if (n == 0) return Ref(node);
-  cord_internal::CordTreeMutablePath rhs_stack;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> rhs_stack;
 
   while (node->tag == CONCAT) {
     assert(n <= node->length);
@@ -959,7 +924,7 @@
 static CordRep* RemoveSuffixFrom(CordRep* node, size_t n) {
   if (n >= node->length) return nullptr;
   if (n == 0) return Ref(node);
-  absl::cord_internal::CordTreeMutablePath lhs_stack;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> lhs_stack;
   bool inplace_ok = node->refcount.IsOne();
 
   while (node->tag == CONCAT) {
@@ -1030,7 +995,6 @@
 
 // Work item for NewSubRange().
 struct SubRange {
-  SubRange() = default;
   SubRange(CordRep* a_node, size_t a_pos, size_t a_n)
       : node(a_node), pos(a_pos), n(a_n) {}
   CordRep* node;  // nullptr means concat last 2 results.
@@ -1039,11 +1003,8 @@
 };
 
 static CordRep* NewSubRange(CordRep* node, size_t pos, size_t n) {
-  cord_internal::CordTreeMutablePath results;
-  // The algorithm below in worst case scenario adds up to 3 nodes to the `todo`
-  // list, but we also pop one out on every cycle. If original tree has depth d
-  // todo list can grew up to 2*d in size.
-  cord_internal::CordTreePath<SubRange, 2 * cord_internal::MaxCordDepth()> todo;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> results;
+  absl::InlinedVector<SubRange, kInlinedVectorSize> todo;
   todo.push_back(SubRange(node, pos, n));
   do {
     const SubRange& sr = todo.back();
@@ -1080,7 +1041,7 @@
     }
   } while (!todo.empty());
   assert(results.size() == 1);
-  return results.back();
+  return results[0];
 }
 
 Cord Cord::Subcord(size_t pos, size_t new_size) const {
@@ -1096,7 +1057,7 @@
   } else if (new_size == 0) {
     // We want to return empty subcord, so nothing to do.
   } else if (new_size <= InlineRep::kMaxInline) {
-    Cord::InternalChunkIterator it = internal_chunk_begin();
+    Cord::ChunkIterator it = chunk_begin();
     it.AdvanceBytes(pos);
     char* dest = sub_cord.contents_.data_;
     size_t remaining_size = new_size;
@@ -1119,12 +1080,11 @@
 
 class CordForest {
  public:
-  explicit CordForest(size_t length) : root_length_(length), trees_({}) {}
+  explicit CordForest(size_t length)
+      : root_length_(length), trees_(kMinLengthSize, nullptr) {}
 
   void Build(CordRep* cord_root) {
-    // We are adding up to two nodes to the `pending` list, but we also popping
-    // one, so the size of `pending` will never exceed `MaxCordDepth()`.
-    cord_internal::CordTreeMutablePath pending(cord_root);
+    std::vector<CordRep*> pending = {cord_root};
 
     while (!pending.empty()) {
       CordRep* node = pending.back();
@@ -1136,20 +1096,21 @@
       }
 
       CordRepConcat* concat_node = node->concat();
-      if (IsNodeBalanced(concat_node)) {
-        AddNode(node);
-        continue;
-      }
-      pending.push_back(concat_node->right);
-      pending.push_back(concat_node->left);
+      if (concat_node->depth() >= kMinLengthSize ||
+          concat_node->length < min_length[concat_node->depth()]) {
+        pending.push_back(concat_node->right);
+        pending.push_back(concat_node->left);
 
-      if (concat_node->refcount.IsOne()) {
-        concat_node->left = concat_freelist_;
-        concat_freelist_ = concat_node;
+        if (concat_node->refcount.IsOne()) {
+          concat_node->left = concat_freelist_;
+          concat_freelist_ = concat_node;
+        } else {
+          Ref(concat_node->right);
+          Ref(concat_node->left);
+          Unref(concat_node);
+        }
       } else {
-        Ref(concat_node->right);
-        Ref(concat_node->left);
-        Unref(concat_node);
+        AddNode(node);
       }
     }
   }
@@ -1181,7 +1142,7 @@
 
     // Collect together everything with which we will merge with node
     int i = 0;
-    for (; node->length >= kMinLength[i + 1]; ++i) {
+    for (; node->length > min_length[i + 1]; ++i) {
       auto& tree_at_i = trees_[i];
 
       if (tree_at_i == nullptr) continue;
@@ -1192,7 +1153,7 @@
     sum = AppendNode(node, sum);
 
     // Insert sum into appropriate place in the forest
-    for (; sum->length >= kMinLength[i]; ++i) {
+    for (; sum->length >= min_length[i]; ++i) {
       auto& tree_at_i = trees_[i];
       if (tree_at_i == nullptr) continue;
 
@@ -1200,7 +1161,7 @@
       tree_at_i = nullptr;
     }
 
-    // kMinLength[0] == 1, which means sum->length >= kMinLength[0]
+    // min_length[0] == 1, which means sum->length >= min_length[0]
     assert(i > 0);
     trees_[i - 1] = sum;
   }
@@ -1233,7 +1194,9 @@
   }
 
   size_t root_length_;
-  std::array<cord_internal::CordRep*, cord_internal::MaxCordDepth()> trees_;
+
+  // use an inlined vector instead of a flat array to get bounds checking
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> trees_;
 
   // List of concat nodes we can re-use for Cord balancing.
   CordRepConcat* concat_freelist_ = nullptr;
@@ -1334,7 +1297,7 @@
 
 inline int Cord::CompareSlowPath(absl::string_view rhs, size_t compared_size,
                                  size_t size_to_compare) const {
-  auto advance = [](Cord::InternalChunkIterator* it, absl::string_view* chunk) {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
     if (!chunk->empty()) return true;
     ++*it;
     if (it->bytes_remaining_ == 0) return false;
@@ -1342,7 +1305,7 @@
     return true;
   };
 
-  Cord::InternalChunkIterator lhs_it = internal_chunk_begin();
+  Cord::ChunkIterator lhs_it = chunk_begin();
 
   // compared_size is inside first chunk.
   absl::string_view lhs_chunk =
@@ -1364,7 +1327,7 @@
 
 inline int Cord::CompareSlowPath(const Cord& rhs, size_t compared_size,
                                  size_t size_to_compare) const {
-  auto advance = [](Cord::InternalChunkIterator* it, absl::string_view* chunk) {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
     if (!chunk->empty()) return true;
     ++*it;
     if (it->bytes_remaining_ == 0) return false;
@@ -1372,8 +1335,8 @@
     return true;
   };
 
-  Cord::InternalChunkIterator lhs_it = internal_chunk_begin();
-  Cord::InternalChunkIterator rhs_it = rhs.internal_chunk_begin();
+  Cord::ChunkIterator lhs_it = chunk_begin();
+  Cord::ChunkIterator rhs_it = rhs.chunk_begin();
 
   // compared_size is inside both first chunks.
   absl::string_view lhs_chunk =
@@ -1507,9 +1470,7 @@
   }
 }
 
-template <typename StorageType>
-Cord::GenericChunkIterator<StorageType>&
-Cord::GenericChunkIterator<StorageType>::operator++() {
+Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
   ABSL_HARDENING_ASSERT(bytes_remaining_ > 0 &&
                         "Attempted to iterate past `end()`");
   assert(bytes_remaining_ >= current_chunk_.size());
@@ -1549,8 +1510,7 @@
   return *this;
 }
 
-template <typename StorageType>
-Cord Cord::GenericChunkIterator<StorageType>::AdvanceAndReadBytes(size_t n) {
+Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
   ABSL_HARDENING_ASSERT(bytes_remaining_ >= n &&
                         "Attempted to iterate past `end()`");
   Cord subcord;
@@ -1664,8 +1624,7 @@
   return subcord;
 }
 
-template <typename StorageType>
-void Cord::GenericChunkIterator<StorageType>::AdvanceBytesSlowPath(size_t n) {
+void Cord::ChunkIterator::AdvanceBytesSlowPath(size_t n) {
   assert(bytes_remaining_ >= n && "Attempted to iterate past `end()`");
   assert(n >= current_chunk_.size());  // This should only be called when
                                        // iterating to a new node.
@@ -1851,18 +1810,18 @@
   }
 }
 
-static void DumpNode(const CordRep* rep, bool include_data, std::ostream* os) {
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os) {
   const int kIndentStep = 1;
   int indent = 0;
-  cord_internal::CordTreeConstPath stack;
-  cord_internal::CordTreePath<int, cord_internal::MaxCordDepth()> indents;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> stack;
+  absl::InlinedVector<int, kInlinedVectorSize> indents;
   for (;;) {
     *os << std::setw(3) << rep->refcount.Get();
     *os << " " << std::setw(7) << rep->length;
     *os << " [";
-    if (include_data) *os << static_cast<const void*>(rep);
+    if (include_data) *os << static_cast<void*>(rep);
     *os << "]";
-    *os << " " << (IsNodeBalanced(rep) ? 'b' : 'u');
+    *os << " " << (IsRootBalanced(rep) ? 'b' : 'u');
     *os << " " << std::setw(indent) << "";
     if (rep->tag == CONCAT) {
       *os << "CONCAT depth=" << Depth(rep) << "\n";
@@ -1883,7 +1842,7 @@
       } else {
         *os << "FLAT cap=" << TagToLength(rep->tag) << " [";
         if (include_data)
-          *os << absl::CEscape(absl::string_view(rep->data, rep->length));
+          *os << absl::CEscape(std::string(rep->data, rep->length));
         *os << "]\n";
       }
       if (stack.empty()) break;
@@ -1896,19 +1855,19 @@
   ABSL_INTERNAL_CHECK(indents.empty(), "");
 }
 
-static std::string ReportError(const CordRep* root, const CordRep* node) {
+static std::string ReportError(CordRep* root, CordRep* node) {
   std::ostringstream buf;
   buf << "Error at node " << node << " in:";
   DumpNode(root, true, &buf);
   return buf.str();
 }
 
-static bool VerifyNode(const CordRep* root, const CordRep* start_node,
+static bool VerifyNode(CordRep* root, CordRep* start_node,
                        bool full_validation) {
-  cord_internal::CordTreeConstPath worklist;
+  absl::InlinedVector<CordRep*, 2> worklist;
   worklist.push_back(start_node);
   do {
-    const CordRep* node = worklist.back();
+    CordRep* node = worklist.back();
     worklist.pop_back();
 
     ABSL_INTERNAL_CHECK(node != nullptr, ReportError(root, node));
@@ -1958,7 +1917,7 @@
   // Iterate over the tree. cur_node is never a leaf node and leaf nodes will
   // never be appended to tree_stack. This reduces overhead from manipulating
   // tree_stack.
-  cord_internal::CordTreeConstPath tree_stack;
+  absl::InlinedVector<const CordRep*, kInlinedVectorSize> tree_stack;
   const CordRep* cur_node = rep;
   while (true) {
     const CordRep* next_node = nullptr;
@@ -2005,9 +1964,6 @@
   return out;
 }
 
-template class Cord::GenericChunkIterator<cord_internal::CordTreeMutablePath>;
-template class Cord::GenericChunkIterator<cord_internal::CordTreeDynamicPath>;
-
 namespace strings_internal {
 size_t CordTestAccess::FlatOverhead() { return kFlatOverhead; }
 size_t CordTestAccess::MaxFlatLength() { return kMaxFlatLength; }

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index 3ab3cb8..ae3d2e7 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h

@@ -90,10 +90,6 @@
 template <typename Releaser>
 Cord MakeCordFromExternal(absl::string_view, Releaser&&);
 void CopyCordToString(const Cord& src, std::string* dst);
-namespace hash_internal {
-template <typename H>
-H HashFragmentedCord(H, const Cord&);
-}
 
 // Cord
 //
@@ -123,132 +119,12 @@
 // Additionally, the API provides iterator utilities to iterate through Cord
 // data via chunks or character bytes.
 //
-
-namespace cord_internal {
-
-// It's expensive to keep a Cord's tree perfectly balanced, so instead we keep
-// trees approximately balanced.  A tree node N of depth D(N) that contains a
-// string of L(N) characters is considered balanced if L >= Fibonacci(D + 2).
-// The "+ 2" is used to ensure that every balanced leaf node contains at least
-//  one character. Here we presume that
-//   Fibonacci(0) = 0
-//   Fibonacci(1) = 1
-//   Fibonacci(2) = 1
-//   Fibonacci(3) = 2
-//   ...
-// The algorithm is based on paper by Hans Boehm et al:
-// https://www.cs.rit.edu/usr/local/pub/jeh/courses/QUARTERS/FP/Labs/CedarRope/rope-paper.pdf
-// In this paper authors shows that rebalancing based on cord forest of already
-// balanced subtrees can be proven to never produce tree of depth larger than
-// largest Fibonacci number representable in the same integral type as cord size
-// For 64 bit integers this is the 93rd Fibonacci number. For 32 bit integrals
-// this is 47th Fibonacci number.
-constexpr size_t MaxCordDepth() { return sizeof(size_t) == 8 ? 93 : 47; }
-
-// This class models fixed max size stack of CordRep pointers.
-// The elements are being pushed back and popped from the back.
-template <typename CordRepPtr, size_t N>
-class CordTreePath {
- public:
-  CordTreePath() {}
-  explicit CordTreePath(CordRepPtr root) { push_back(root); }
-
-  bool empty() const { return size_ == 0; }
-  size_t size() const { return size_; }
-  void clear() { size_ = 0; }
-
-  CordRepPtr back() { return data_[size_ - 1]; }
-
-  void pop_back() {
-    --size_;
-    assert(size_ < N);
-  }
-  void push_back(CordRepPtr elem) { data_[size_++] = elem; }
-
- private:
-  CordRepPtr data_[N];
-  size_t size_ = 0;
-};
-
-// Fixed length container for mutable "path" in cord tree, which can hold any
-// possible valid path in cord tree.
-using CordTreeMutablePath = CordTreePath<CordRep*, MaxCordDepth()>;
-// Variable length container for mutable "path" in cord tree. It starts with
-// capacity for 15 elements and grow if necessary.
-using CordTreeDynamicPath =
-    absl::InlinedVector<absl::cord_internal::CordRep*, 15>;
-}  // namespace cord_internal
-
-// A Cord is a sequence of characters.
 class Cord {
  private:
   template <typename T>
   using EnableIfString =
       absl::enable_if_t<std::is_same<T, std::string>::value, int>;
 
-  //----------------------------------------------------------------------------
-  // Cord::GenericChunkIterator
-  //----------------------------------------------------------------------------
-  //
-  // A `Cord::GenericChunkIterator` provides an interface for the standard
-  // `Cord::ChunkIterator` as well as some private implementations.
-  template <typename StorageType>
-  class GenericChunkIterator {
-   public:
-    using iterator_category = std::input_iterator_tag;
-    using value_type = absl::string_view;
-    using difference_type = ptrdiff_t;
-    using pointer = const value_type*;
-    using reference = value_type;
-
-    GenericChunkIterator() = default;
-
-    GenericChunkIterator& operator++();
-    GenericChunkIterator operator++(int);
-    bool operator==(const GenericChunkIterator& other) const;
-    bool operator!=(const GenericChunkIterator& other) const;
-    reference operator*() const;
-    pointer operator->() const;
-
-    friend class Cord;
-    friend class CharIterator;
-
-   private:
-    // Constructs a `begin()` iterator from `cord`.
-    explicit GenericChunkIterator(const Cord* cord);
-
-    // Removes `n` bytes from `current_chunk_`. Expects `n` to be smaller than
-    // `current_chunk_.size()`.
-    void RemoveChunkPrefix(size_t n);
-    Cord AdvanceAndReadBytes(size_t n);
-    void AdvanceBytes(size_t n);
-    // Iterates `n` bytes, where `n` is expected to be greater than or equal to
-    // `current_chunk_.size()`.
-    void AdvanceBytesSlowPath(size_t n);
-
-    // A view into bytes of the current `CordRep`. It may only be a view to a
-    // suffix of bytes if this is being used by `CharIterator`.
-    absl::string_view current_chunk_;
-    // The current leaf, or `nullptr` if the iterator points to short data.
-    // If the current chunk is a substring node, current_leaf_ points to the
-    // underlying flat or external node.
-    cord_internal::CordRep* current_leaf_ = nullptr;
-    // The number of bytes left in the `Cord` over which we are iterating.
-    size_t bytes_remaining_ = 0;
-    StorageType stack_of_right_children_;
-  };
-  template <typename IteratorType>
-  class GenericChunkRange {
-   public:
-    explicit GenericChunkRange(const Cord* cord) : cord_(cord) {}
-
-    IteratorType begin() const { return IteratorType(cord_); }
-    IteratorType end() const { return IteratorType(); }
-
-   private:
-    const Cord* cord_;
-  };
-
  public:
   // Cord::Cord() Constructors
 
@@ -385,7 +261,7 @@
   // Determines whether the given Cord is empty, returning `true` is so.
   bool empty() const;
 
-  // Cord:EstimatedMemoryUsage()
+  // Cord::EstimatedMemoryUsage()
   //
   // Returns the *approximate* number of bytes held in full or in part by this
   // Cord (which may not remain the same between invocations).  Note that Cords
@@ -464,8 +340,51 @@
   //   * The iterator keeps state that can grow for Cords that contain many
   //     nodes and are imbalanced due to sharing. Prefer to pass this type by
   //     const reference instead of by value.
-  using ChunkIterator =
-      GenericChunkIterator<cord_internal::CordTreeDynamicPath>;
+  class ChunkIterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = absl::string_view;
+    using difference_type = ptrdiff_t;
+    using pointer = const value_type*;
+    using reference = value_type;
+
+    ChunkIterator() = default;
+
+    ChunkIterator& operator++();
+    ChunkIterator operator++(int);
+    bool operator==(const ChunkIterator& other) const;
+    bool operator!=(const ChunkIterator& other) const;
+    reference operator*() const;
+    pointer operator->() const;
+
+    friend class Cord;
+    friend class CharIterator;
+
+   private:
+    // Constructs a `begin()` iterator from `cord`.
+    explicit ChunkIterator(const Cord* cord);
+
+    // Removes `n` bytes from `current_chunk_`. Expects `n` to be smaller than
+    // `current_chunk_.size()`.
+    void RemoveChunkPrefix(size_t n);
+    Cord AdvanceAndReadBytes(size_t n);
+    void AdvanceBytes(size_t n);
+    // Iterates `n` bytes, where `n` is expected to be greater than or equal to
+    // `current_chunk_.size()`.
+    void AdvanceBytesSlowPath(size_t n);
+
+    // A view into bytes of the current `CordRep`. It may only be a view to a
+    // suffix of bytes if this is being used by `CharIterator`.
+    absl::string_view current_chunk_;
+    // The current leaf, or `nullptr` if the iterator points to short data.
+    // If the current chunk is a substring node, current_leaf_ points to the
+    // underlying flat or external node.
+    absl::cord_internal::CordRep* current_leaf_ = nullptr;
+    // The number of bytes left in the `Cord` over which we are iterating.
+    size_t bytes_remaining_ = 0;
+    absl::InlinedVector<absl::cord_internal::CordRep*, 4>
+        stack_of_right_children_;
+  };
 
   // Cord::ChunkIterator::chunk_begin()
   //
@@ -504,7 +423,16 @@
   //
   // Implementation note: `ChunkRange` is simply a convenience wrapper over
   // `Cord::chunk_begin()` and `Cord::chunk_end()`.
-  using ChunkRange = GenericChunkRange<ChunkIterator>;
+  class ChunkRange {
+   public:
+    explicit ChunkRange(const Cord* cord) : cord_(cord) {}
+
+    ChunkIterator begin() const;
+    ChunkIterator end() const;
+
+   private:
+    const Cord* cord_;
+  };
 
   // Cord::Chunks()
   //
@@ -683,10 +611,22 @@
   // If the cord was already flat, the contents are not modified.
   absl::string_view Flatten();
 
+  // Support absl::Cord as a sink object for absl::Format().
+  friend void AbslFormatFlush(absl::Cord* cord, absl::string_view part) {
+    cord->Append(part);
+  }
+
+  template <typename H>
+  friend H AbslHashValue(H hash_state, const absl::Cord& c) {
+    absl::optional<absl::string_view> maybe_flat = c.TryFlat();
+    if (maybe_flat.has_value()) {
+      return H::combine(std::move(hash_state), *maybe_flat);
+    }
+    return c.HashFragmented(std::move(hash_state));
+  }
+
  private:
   friend class CordTestPeer;
-  template <typename H>
-  friend H absl::hash_internal::HashFragmentedCord(H, const Cord&);
   friend bool operator==(const Cord& lhs, const Cord& rhs);
   friend bool operator==(const Cord& lhs, absl::string_view rhs);
 
@@ -702,16 +642,15 @@
   // class so that we can isolate the bulk of cord.cc from changes
   // to the representation.
   //
-  // InlineRep holds either either a tree pointer, or an array of kMaxInline
-  // bytes.
+  // InlineRep holds either a tree pointer, or an array of kMaxInline bytes.
   class InlineRep {
    public:
-    static const unsigned char kMaxInline = 15;
+    static constexpr unsigned char kMaxInline = 15;
     static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*), "");
     // Tag byte & kMaxInline means we are storing a pointer.
-    static const unsigned char kTreeFlag = 1 << 4;
+    static constexpr unsigned char kTreeFlag = 1 << 4;
     // Tag byte & kProfiledFlag means we are profiling the Cord.
-    static const unsigned char kProfiledFlag = 1 << 5;
+    static constexpr unsigned char kProfiledFlag = 1 << 5;
 
     constexpr InlineRep() : data_{} {}
     InlineRep(const InlineRep& src);
@@ -800,14 +739,6 @@
   static bool GetFlatAux(absl::cord_internal::CordRep* rep,
                          absl::string_view* fragment);
 
-  // Iterators for use inside Cord implementation
-  using InternalChunkIterator =
-      GenericChunkIterator<cord_internal::CordTreeMutablePath>;
-  using InternalChunkRange = GenericChunkRange<InternalChunkIterator>;
-
-  InternalChunkIterator internal_chunk_begin() const;
-  InternalChunkRange InternalChunks() const;
-
   // Helper for ForEachChunk()
   static void ForEachChunkAux(
       absl::cord_internal::CordRep* rep,
@@ -840,12 +771,18 @@
   // Helper for Append()
   template <typename C>
   void AppendImpl(C&& src);
-};
 
-extern template class Cord::GenericChunkIterator<
-    cord_internal::CordTreeMutablePath>;
-extern template class Cord::GenericChunkIterator<
-    cord_internal::CordTreeDynamicPath>;
+  // Helper for AbslHashValue()
+  template <typename H>
+  H HashFragmented(H hash_state) const {
+    typename H::AbslInternalPiecewiseCombiner combiner;
+    ForEachChunk([&combiner, &hash_state](absl::string_view chunk) {
+      hash_state = combiner.add_buffer(std::move(hash_state), chunk.data(),
+                                       chunk.size());
+    });
+    return H::combine(combiner.finalize(std::move(hash_state)), size());
+  }
+};
 
 ABSL_NAMESPACE_END
 }  // namespace absl
@@ -1186,9 +1123,7 @@
   return EqualsImpl(rhs, rhs_size);
 }
 
-template <typename StorageType>
-inline Cord::GenericChunkIterator<StorageType>::GenericChunkIterator(
-    const Cord* cord)
+inline Cord::ChunkIterator::ChunkIterator(const Cord* cord)
     : bytes_remaining_(cord->size()) {
   if (cord->empty()) return;
   if (cord->contents_.is_tree()) {
@@ -1199,50 +1134,37 @@
   }
 }
 
-template <typename StorageType>
-inline Cord::GenericChunkIterator<StorageType>
-Cord::GenericChunkIterator<StorageType>::operator++(int) {
-  GenericChunkIterator tmp(*this);
+inline Cord::ChunkIterator Cord::ChunkIterator::operator++(int) {
+  ChunkIterator tmp(*this);
   operator++();
   return tmp;
 }
 
-template <typename StorageType>
-inline bool Cord::GenericChunkIterator<StorageType>::operator==(
-    const GenericChunkIterator<StorageType>& other) const {
+inline bool Cord::ChunkIterator::operator==(const ChunkIterator& other) const {
   return bytes_remaining_ == other.bytes_remaining_;
 }
 
-template <typename StorageType>
-inline bool Cord::GenericChunkIterator<StorageType>::operator!=(
-    const GenericChunkIterator<StorageType>& other) const {
+inline bool Cord::ChunkIterator::operator!=(const ChunkIterator& other) const {
   return !(*this == other);
 }
 
-template <typename StorageType>
-inline typename Cord::GenericChunkIterator<StorageType>::reference
-Cord::GenericChunkIterator<StorageType>::operator*() const {
+inline Cord::ChunkIterator::reference Cord::ChunkIterator::operator*() const {
   ABSL_HARDENING_ASSERT(bytes_remaining_ != 0);
   return current_chunk_;
 }
 
-template <typename StorageType>
-inline typename Cord::GenericChunkIterator<StorageType>::pointer
-Cord::GenericChunkIterator<StorageType>::operator->() const {
+inline Cord::ChunkIterator::pointer Cord::ChunkIterator::operator->() const {
   ABSL_HARDENING_ASSERT(bytes_remaining_ != 0);
   return &current_chunk_;
 }
 
-template <typename StorageType>
-inline void Cord::GenericChunkIterator<StorageType>::RemoveChunkPrefix(
-    size_t n) {
+inline void Cord::ChunkIterator::RemoveChunkPrefix(size_t n) {
   assert(n < current_chunk_.size());
   current_chunk_.remove_prefix(n);
   bytes_remaining_ -= n;
 }
 
-template <typename StorageType>
-inline void Cord::GenericChunkIterator<StorageType>::AdvanceBytes(size_t n) {
+inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
   if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
     RemoveChunkPrefix(n);
   } else if (n != 0) {
@@ -1256,6 +1178,14 @@
 
 inline Cord::ChunkIterator Cord::chunk_end() const { return ChunkIterator(); }
 
+inline Cord::ChunkIterator Cord::ChunkRange::begin() const {
+  return cord_->chunk_begin();
+}
+
+inline Cord::ChunkIterator Cord::ChunkRange::end() const {
+  return cord_->chunk_end();
+}
+
 inline Cord::ChunkRange Cord::Chunks() const { return ChunkRange(this); }
 
 inline Cord::CharIterator& Cord::CharIterator::operator++() {

diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
index 0ec93b4..336cedd 100644
--- a/absl/strings/cord_test.cc
+++ b/absl/strings/cord_test.cc

@@ -22,6 +22,7 @@
 #include "absl/container/fixed_array.h"
 #include "absl/strings/cord_test_helpers.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
 #include "absl/strings/string_view.h"
 
 typedef std::mt19937_64 RandomEngine;
@@ -1403,53 +1404,6 @@
   VerifyChunkIterator(subcords, 128);
 }
 
-TEST(CordChunkIterator, MaxLengthFullTree) {
-  // Start with a 1-byte cord, and then double its length in a loop.  We should
-  // be able to do this until the point where we would overflow size_t.
-
-  absl::Cord cord;
-  size_t size = 1;
-  AddExternalMemory("x", &cord);
-  EXPECT_EQ(cord.size(), size);
-
-  const int kCordLengthDoublingLimit = std::numeric_limits<size_t>::digits - 1;
-  for (int i = 0; i < kCordLengthDoublingLimit; ++i) {
-    cord.Prepend(absl::Cord(cord));
-    size <<= 1;
-
-    EXPECT_EQ(cord.size(), size);
-
-    auto chunk_it = cord.chunk_begin();
-    EXPECT_EQ(*chunk_it, "x");
-  }
-
-  EXPECT_DEATH_IF_SUPPORTED(
-      (cord.Prepend(absl::Cord(cord)), *cord.chunk_begin()),
-      "Cord is too long");
-}
-
-TEST(CordChunkIterator, MaxDepth) {
-  // By reusing nodes, it's possible in pathological cases to build a Cord that
-  // exceeds both the maximum permissible length and depth.  In this case, the
-  // violation of the maximum depth is reported.
-  absl::Cord left_child;
-  AddExternalMemory("x", &left_child);
-  absl::Cord root = left_child;
-
-  for (int i = 0; i < absl::cord_internal::MaxCordDepth() - 2; ++i) {
-    size_t new_size = left_child.size() + root.size();
-    root.Prepend(left_child);
-    EXPECT_EQ(root.size(), new_size);
-
-    auto chunk_it = root.chunk_begin();
-    EXPECT_EQ(*chunk_it, "x");
-
-    std::swap(left_child, root);
-  }
-
-  EXPECT_DEATH_IF_SUPPORTED(root.Prepend(left_child), "Cord is too long");
-}
-
 TEST(CordCharIterator, Traits) {
   static_assert(std::is_copy_constructible<absl::Cord::CharIterator>::value,
                 "");
@@ -1629,6 +1583,14 @@
   }
 }
 
+TEST(Cord, Format) {
+  absl::Cord c;
+  absl::Format(&c, "There were %04d little %s.", 3, "pigs");
+  EXPECT_EQ(c, "There were 0003 little pigs.");
+  absl::Format(&c, "And %-3llx bad wolf!", 1);
+  EXPECT_EQ(c, "There were 0003 little pigs.And 1   bad wolf!");
+}
+
 TEST(CordDeathTest, Hardening) {
   absl::Cord cord("hello");
   // These statement should abort the program in all builds modes.

diff --git a/absl/strings/internal/str_format/arg.cc b/absl/strings/internal/str_format/arg.cc
index 3aa0296..1284227 100644
--- a/absl/strings/internal/str_format/arg.cc
+++ b/absl/strings/internal/str_format/arg.cc

@@ -12,14 +12,13 @@
 
 #include "absl/base/port.h"
 #include "absl/strings/internal/str_format/float_conversion.h"
+#include "absl/strings/numbers.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
-const char kDigit[2][32] = { "0123456789abcdef", "0123456789ABCDEF" };
-
 // Reduce *capacity by s.size(), clipped to a 0 minimum.
 void ReducePadding(string_view s, size_t *capacity) {
   *capacity = Excess(s.size(), *capacity);
@@ -48,94 +47,144 @@
 template <>
 struct IsSigned<absl::uint128> : std::false_type {};
 
-class ConvertedIntInfo {
+// Integral digit printer.
+// Call one of the PrintAs* routines after construction once.
+// Use with_neg_and_zero/without_neg_or_zero/is_negative to access the results.
+class IntDigits {
  public:
+  // Print the unsigned integer as octal.
+  // Supports unsigned integral types and uint128.
   template <typename T>
-  ConvertedIntInfo(T v, ConversionChar conv) {
-    using Unsigned = typename MakeUnsigned<T>::type;
-    auto u = static_cast<Unsigned>(v);
-    if (IsNeg(v)) {
-      is_neg_ = true;
-      u = Unsigned{} - u;
-    } else {
-      is_neg_ = false;
-    }
-    UnsignedToStringRight(u, conv);
+  void PrintAsOct(T v) {
+    static_assert(!IsSigned<T>::value, "");
+    char *p = storage_ + sizeof(storage_);
+    do {
+      *--p = static_cast<char>('0' + (static_cast<size_t>(v) & 7));
+      v >>= 3;
+    } while (v);
+    start_ = p;
+    size_ = storage_ + sizeof(storage_) - p;
   }
 
-  string_view digits() const {
-    return {end() - size_, static_cast<size_t>(size_)};
+  // Print the signed or unsigned integer as decimal.
+  // Supports all integral types.
+  template <typename T>
+  void PrintAsDec(T v) {
+    static_assert(std::is_integral<T>::value, "");
+    start_ = storage_;
+    size_ = numbers_internal::FastIntToBuffer(v, storage_) - storage_;
   }
-  bool is_neg() const { return is_neg_; }
+
+  void PrintAsDec(int128 v) {
+    auto u = static_cast<uint128>(v);
+    bool add_neg = false;
+    if (v < 0) {
+      add_neg = true;
+      u = uint128{} - u;
+    }
+    PrintAsDec(u, add_neg);
+  }
+
+  void PrintAsDec(uint128 v, bool add_neg = false) {
+    // This function can be sped up if needed. We can call FastIntToBuffer
+    // twice, or fix FastIntToBuffer to support uint128.
+    char *p = storage_ + sizeof(storage_);
+    do {
+      p -= 2;
+      numbers_internal::PutTwoDigits(static_cast<size_t>(v % 100), p);
+      v /= 100;
+    } while (v);
+    if (p[0] == '0') {
+      // We printed one too many hexits.
+      ++p;
+    }
+    if (add_neg) {
+      *--p = '-';
+    }
+    size_ = storage_ + sizeof(storage_) - p;
+    start_ = p;
+  }
+
+  // Print the unsigned integer as hex using lowercase.
+  // Supports unsigned integral types and uint128.
+  template <typename T>
+  void PrintAsHexLower(T v) {
+    static_assert(!IsSigned<T>::value, "");
+    char *p = storage_ + sizeof(storage_);
+
+    do {
+      p -= 2;
+      constexpr const char* table = numbers_internal::kHexTable;
+      std::memcpy(p, table + 2 * (static_cast<size_t>(v) & 0xFF), 2);
+      if (sizeof(T) == 1) break;
+      v >>= 8;
+    } while (v);
+    if (p[0] == '0') {
+      // We printed one too many digits.
+      ++p;
+    }
+    start_ = p;
+    size_ = storage_ + sizeof(storage_) - p;
+  }
+
+  // Print the unsigned integer as hex using uppercase.
+  // Supports unsigned integral types and uint128.
+  template <typename T>
+  void PrintAsHexUpper(T v) {
+    static_assert(!IsSigned<T>::value, "");
+    char *p = storage_ + sizeof(storage_);
+
+    // kHexTable is only lowercase, so do it manually for uppercase.
+    do {
+      *--p = "0123456789ABCDEF"[static_cast<size_t>(v) & 15];
+      v >>= 4;
+    } while (v);
+    start_ = p;
+    size_ = storage_ + sizeof(storage_) - p;
+  }
+
+  // The printed value including the '-' sign if available.
+  // For inputs of value `0`, this will return "0"
+  string_view with_neg_and_zero() const { return {start_, size_}; }
+
+  // The printed value not including the '-' sign.
+  // For inputs of value `0`, this will return "".
+  string_view without_neg_or_zero() const {
+    static_assert('-' < '0', "The check below verifies both.");
+    size_t advance = start_[0] <= '0' ? 1 : 0;
+    return {start_ + advance, size_ - advance};
+  }
+
+  bool is_negative() const { return start_[0] == '-'; }
 
  private:
-  template <typename T, bool IsSigned>
-  struct IsNegImpl {
-    static bool Eval(T v) { return v < 0; }
-  };
-  template <typename T>
-  struct IsNegImpl<T, false> {
-    static bool Eval(T) {
-      return false;
-    }
-  };
-
-  template <typename T>
-  bool IsNeg(T v) {
-    return IsNegImpl<T, IsSigned<T>::value>::Eval(v);
-  }
-
-  template <typename T>
-  void UnsignedToStringRight(T u, ConversionChar conv) {
-    char *p = end();
-    switch (FormatConversionCharRadix(conv)) {
-      default:
-      case 10:
-        for (; u; u /= 10)
-          *--p = static_cast<char>('0' + static_cast<size_t>(u % 10));
-        break;
-      case 8:
-        for (; u; u /= 8)
-          *--p = static_cast<char>('0' + static_cast<size_t>(u % 8));
-        break;
-      case 16: {
-        const char *digits = kDigit[FormatConversionCharIsUpper(conv) ? 1 : 0];
-        for (; u; u /= 16) *--p = digits[static_cast<size_t>(u % 16)];
-        break;
-      }
-    }
-    size_ = static_cast<int>(end() - p);
-  }
-
-  const char *end() const { return storage_ + sizeof(storage_); }
-  char *end() { return storage_ + sizeof(storage_); }
-
-  bool is_neg_;
-  int size_;
-  // Max size: 128 bit value as octal -> 43 digits
-  char storage_[128 / 3 + 1];
+  const char *start_;
+  size_t size_;
+  // Max size: 128 bit value as octal -> 43 digits, plus sign char
+  char storage_[128 / 3 + 1 + 1];
 };
 
 // Note: 'o' conversions do not have a base indicator, it's just that
 // the '#' flag is specified to modify the precision for 'o' conversions.
-string_view BaseIndicator(const ConvertedIntInfo &info,
+string_view BaseIndicator(const IntDigits &as_digits,
                           const ConversionSpec conv) {
-  bool alt = conv.has_alt_flag();
-  int radix = FormatConversionCharRadix(conv.conversion_char());
-  if (conv.conversion_char() == ConversionChar::p)
-    alt = true;  // always show 0x for %p.
+  // always show 0x for %p.
+  bool alt = conv.has_alt_flag() || conv.conversion_char() == ConversionChar::p;
+  bool hex = (conv.conversion_char() == FormatConversionChar::x ||
+              conv.conversion_char() == FormatConversionChar::X ||
+              conv.conversion_char() == FormatConversionChar::p);
   // From the POSIX description of '#' flag:
   //   "For x or X conversion specifiers, a non-zero result shall have
   //   0x (or 0X) prefixed to it."
-  if (alt && radix == 16 && !info.digits().empty()) {
-    if (FormatConversionCharIsUpper(conv.conversion_char())) return "0X";
-    return "0x";
+  if (alt && hex && !as_digits.without_neg_or_zero().empty()) {
+    return conv.conversion_char() == FormatConversionChar::X ? "0X" : "0x";
   }
   return {};
 }
 
 string_view SignColumn(bool neg, const ConversionSpec conv) {
-  if (FormatConversionCharIsSigned(conv.conversion_char())) {
+  if (conv.conversion_char() == FormatConversionChar::d ||
+      conv.conversion_char() == FormatConversionChar::i) {
     if (neg) return "-";
     if (conv.has_show_pos_flag()) return "+";
     if (conv.has_sign_col_flag()) return " ";
@@ -154,20 +203,20 @@
   return true;
 }
 
-bool ConvertIntImplInner(const ConvertedIntInfo &info,
-                         const ConversionSpec conv, FormatSinkImpl *sink) {
+bool ConvertIntImplInnerSlow(const IntDigits &as_digits,
+                             const ConversionSpec conv, FormatSinkImpl *sink) {
   // Print as a sequence of Substrings:
   //   [left_spaces][sign][base_indicator][zeroes][formatted][right_spaces]
   size_t fill = 0;
   if (conv.width() >= 0) fill = conv.width();
 
-  string_view formatted = info.digits();
+  string_view formatted = as_digits.without_neg_or_zero();
   ReducePadding(formatted, &fill);
 
-  string_view sign = SignColumn(info.is_neg(), conv);
+  string_view sign = SignColumn(as_digits.is_negative(), conv);
   ReducePadding(sign, &fill);
 
-  string_view base_indicator = BaseIndicator(info, conv);
+  string_view base_indicator = BaseIndicator(as_digits, conv);
   ReducePadding(base_indicator, &fill);
 
   int precision = conv.precision();
@@ -209,34 +258,53 @@
 }
 
 template <typename T>
-bool ConvertIntImplInner(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  ConvertedIntInfo info(v, conv.conversion_char());
-  if (conv.is_basic() && (conv.conversion_char() != ConversionChar::p)) {
-    if (info.is_neg()) sink->Append(1, '-');
-    if (info.digits().empty()) {
-      sink->Append(1, '0');
-    } else {
-      sink->Append(info.digits());
-    }
+bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
+  using U = typename MakeUnsigned<T>::type;
+  IntDigits as_digits;
+
+  switch (conv.conversion_char()) {
+    case FormatConversionChar::c:
+      return ConvertCharImpl(static_cast<unsigned char>(v), conv, sink);
+
+    case FormatConversionChar::o:
+      as_digits.PrintAsOct(static_cast<U>(v));
+      break;
+
+    case FormatConversionChar::x:
+      as_digits.PrintAsHexLower(static_cast<U>(v));
+      break;
+    case FormatConversionChar::X:
+      as_digits.PrintAsHexUpper(static_cast<U>(v));
+      break;
+
+    case FormatConversionChar::u:
+      as_digits.PrintAsDec(static_cast<U>(v));
+      break;
+
+    case FormatConversionChar::d:
+    case FormatConversionChar::i:
+      as_digits.PrintAsDec(v);
+      break;
+
+    case FormatConversionChar::a:
+    case FormatConversionChar::e:
+    case FormatConversionChar::f:
+    case FormatConversionChar::g:
+    case FormatConversionChar::A:
+    case FormatConversionChar::E:
+    case FormatConversionChar::F:
+    case FormatConversionChar::G:
+      return ConvertFloatImpl(static_cast<double>(v), conv, sink);
+
+    default:
+      return false;
+  }
+
+  if (conv.is_basic()) {
+    sink->Append(as_digits.with_neg_and_zero());
     return true;
   }
-  return ConvertIntImplInner(info, conv, sink);
-}
-
-template <typename T>
-bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  if (FormatConversionCharIsFloat(conv.conversion_char())) {
-    return FormatConvertImpl(static_cast<double>(v), conv, sink).value;
-  }
-  if (conv.conversion_char() == ConversionChar::c)
-    return ConvertCharImpl(static_cast<unsigned char>(v), conv, sink);
-  if (!FormatConversionCharIsIntegral(conv.conversion_char())) return false;
-  if (!FormatConversionCharIsSigned(conv.conversion_char()) &&
-      IsSigned<T>::value) {
-    using U = typename MakeUnsigned<T>::type;
-    return FormatConvertImpl(static_cast<U>(v), conv, sink).value;
-  }
-  return ConvertIntImplInner(v, conv, sink);
+  return ConvertIntImplInnerSlow(as_digits, conv, sink);
 }
 
 template <typename T>
@@ -296,7 +364,9 @@
     sink->Append("(nil)");
     return {true};
   }
-  return {ConvertIntImplInner(v.value, conv, sink)};
+  IntDigits as_digits;
+  as_digits.PrintAsHexLower(v.value);
+  return {ConvertIntImplInnerSlow(as_digits, conv, sink)};
 }
 
 // ==================== Floats ====================

diff --git a/absl/strings/internal/str_format/bind.h b/absl/strings/internal/str_format/bind.h
index ee4475e..d30fdf5 100644
--- a/absl/strings/internal/str_format/bind.h
+++ b/absl/strings/internal/str_format/bind.h

@@ -60,7 +60,7 @@
   size_t size_;
 };
 
-template <typename T, typename...>
+template <typename T, FormatConversionCharSet...>
 struct MakeDependent {
   using type = T;
 };
@@ -68,7 +68,7 @@
 // Implicitly convertible from `const char*`, `string_view`, and the
 // `ExtendedParsedFormat` type. This abstraction allows all format functions to
 // operate on any without providing too many overloads.
-template <typename... Args>
+template <FormatConversionCharSet... Args>
 class FormatSpecTemplate
     : public MakeDependent<UntypedFormatSpec, Args...>::type {
   using Base = typename MakeDependent<UntypedFormatSpec, Args...>::type;
@@ -105,13 +105,11 @@
 
   // Good format overload.
   FormatSpecTemplate(const char* s)  // NOLINT
-      __attribute__((enable_if(ValidFormatImpl<ArgumentToConv<Args>()...>(s),
-                               "bad format trap")))
+      __attribute__((enable_if(ValidFormatImpl<Args...>(s), "bad format trap")))
       : Base(s) {}
 
   FormatSpecTemplate(string_view s)  // NOLINT
-      __attribute__((enable_if(ValidFormatImpl<ArgumentToConv<Args>()...>(s),
-                               "bad format trap")))
+      __attribute__((enable_if(ValidFormatImpl<Args...>(s), "bad format trap")))
       : Base(s) {}
 
 #else  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
@@ -121,19 +119,14 @@
 
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
-  template <Conv... C, typename = typename std::enable_if<
-                           AllOf(sizeof...(C) == sizeof...(Args),
-                             Contains(ArgumentToConv<Args>(),
-                                          C)...)>::type>
+  template <Conv... C,
+            typename = typename std::enable_if<
+                AllOf(sizeof...(C) == sizeof...(Args), Contains(Args,
+                                                                C)...)>::type>
   FormatSpecTemplate(const ExtendedParsedFormat<C...>& pc)  // NOLINT
       : Base(&pc) {}
 };
 
-template <typename... Args>
-struct FormatSpecDeductionBarrier {
-  using type = FormatSpecTemplate<Args...>;
-};
-
 class Streamable {
  public:
   Streamable(const UntypedFormatSpecImpl& format,

diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index bae2c07..fb31a9d 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h

@@ -170,21 +170,6 @@
   return FormatConversionChar::kNone;
 }
 
-inline int FormatConversionCharRadix(FormatConversionChar c) {
-  switch (c) {
-    case FormatConversionChar::x:
-    case FormatConversionChar::X:
-    case FormatConversionChar::a:
-    case FormatConversionChar::A:
-    case FormatConversionChar::p:
-      return 16;
-    case FormatConversionChar::o:
-      return 8;
-    default:
-      return 10;
-  }
-}
-
 inline bool FormatConversionCharIsUpper(FormatConversionChar c) {
   switch (c) {
     case FormatConversionChar::X:
@@ -198,30 +183,6 @@
   }
 }
 
-inline bool FormatConversionCharIsSigned(FormatConversionChar c) {
-  switch (c) {
-    case FormatConversionChar::d:
-    case FormatConversionChar::i:
-      return true;
-    default:
-      return false;
-  }
-}
-
-inline bool FormatConversionCharIsIntegral(FormatConversionChar c) {
-  switch (c) {
-    case FormatConversionChar::d:
-    case FormatConversionChar::i:
-    case FormatConversionChar::u:
-    case FormatConversionChar::o:
-    case FormatConversionChar::x:
-    case FormatConversionChar::X:
-      return true;
-    default:
-      return false;
-  }
-}
-
 inline bool FormatConversionCharIsFloat(FormatConversionChar c) {
   switch (c) {
     case FormatConversionChar::a:

diff --git a/absl/strings/internal/str_format/extension_test.cc b/absl/strings/internal/str_format/extension_test.cc
index 4e23fef..561eaa3 100644
--- a/absl/strings/internal/str_format/extension_test.cc
+++ b/absl/strings/internal/str_format/extension_test.cc

@@ -19,9 +19,26 @@
 #include <random>
 #include <string>
 
-#include "absl/strings/str_format.h"
-
 #include "gtest/gtest.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/string_view.h"
+
+namespace my_namespace {
+class UserDefinedType {
+ public:
+  UserDefinedType() = default;
+
+  void Append(absl::string_view str) { value_.append(str.data(), str.size()); }
+  const std::string& Value() const { return value_; }
+
+  friend void AbslFormatFlush(UserDefinedType* x, absl::string_view str) {
+    x->Append(str);
+  }
+
+ private:
+  std::string value_;
+};
+}  // namespace my_namespace
 
 namespace {
 
@@ -63,4 +80,13 @@
     EXPECT_EQ(actual, expected);
   }
 }
+
+TEST(FormatExtensionTest, CustomSink) {
+  my_namespace::UserDefinedType sink;
+  absl::Format(&sink, "There were %04d little %s.", 3, "pigs");
+  EXPECT_EQ("There were 0003 little pigs.", sink.Value());
+  absl::Format(&sink, "And %-3llx bad wolf!", 1);
+  EXPECT_EQ("There were 0003 little pigs.And 1   bad wolf!", sink.Value());
+}
+
 }  // namespace

diff --git a/absl/strings/internal/str_format/output.h b/absl/strings/internal/str_format/output.h
index 28b288b..8030dae 100644
--- a/absl/strings/internal/str_format/output.h
+++ b/absl/strings/internal/str_format/output.h

@@ -30,9 +30,6 @@
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
-
-class Cord;
-
 namespace str_format_internal {
 
 // RawSink implementation that writes into a char* buffer.
@@ -77,12 +74,6 @@
   out->write(s.data(), s.size());
 }
 
-template <class AbslCord, typename = typename std::enable_if<
-                              std::is_same<AbslCord, absl::Cord>::value>::type>
-inline void AbslFormatFlush(AbslCord* out, string_view s) {
-  out->Append(s);
-}
-
 inline void AbslFormatFlush(FILERawSink* sink, string_view v) {
   sink->Write(v);
 }
@@ -91,10 +82,11 @@
   sink->Write(v);
 }
 
+// This is a SFINAE to get a better compiler error message when the type
+// is not supported.
 template <typename T>
-auto InvokeFlush(T* out, string_view s)
-    -> decltype(str_format_internal::AbslFormatFlush(out, s)) {
-  str_format_internal::AbslFormatFlush(out, s);
+auto InvokeFlush(T* out, string_view s) -> decltype(AbslFormatFlush(out, s)) {
+  AbslFormatFlush(out, s);
 }
 
 }  // namespace str_format_internal

diff --git a/absl/strings/internal/str_format/output_test.cc b/absl/strings/internal/str_format/output_test.cc
index e54e6f7..ce2e91a 100644
--- a/absl/strings/internal/str_format/output_test.cc
+++ b/absl/strings/internal/str_format/output_test.cc

@@ -19,6 +19,7 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/strings/cord.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -37,6 +38,12 @@
   EXPECT_EQ(str.str(), "ABCDEF");
 }
 
+TEST(InvokeFlush, Cord) {
+  absl::Cord str("ABC");
+  str_format_internal::InvokeFlush(&str, "DEF");
+  EXPECT_EQ(str, "ABCDEF");
+}
+
 TEST(BufferRawSink, Limits) {
   char buf[16];
   {
@@ -70,4 +77,3 @@
 }  // namespace
 ABSL_NAMESPACE_END
 }  // namespace absl
-

diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index d40fca1..f48510b 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h

@@ -57,8 +57,7 @@
 // arbitrary sink types:
 //
 //   * A generic `Format()` function to write outputs to arbitrary sink types,
-//     which must implement a `RawSinkFormat` interface. (See
-//     `str_format_sink.h` for more information.)
+//     which must implement a `FormatRawSink` interface.
 //
 //   * A `FormatUntyped()` function that is similar to `Format()` except it is
 //     loosely typed. `FormatUntyped()` is not a template and does not perform
@@ -255,8 +254,8 @@
 // argument, etc.
 
 template <typename... Args>
-using FormatSpec =
-    typename str_format_internal::FormatSpecDeductionBarrier<Args...>::type;
+using FormatSpec = str_format_internal::FormatSpecTemplate<
+    str_format_internal::ArgumentToConv<Args>()...>;
 
 // ParsedFormat
 //
@@ -432,6 +431,16 @@
 //
 // FormatRawSink is a type erased wrapper around arbitrary sink objects
 // specifically used as an argument to `Format()`.
+//
+// All the object has to do define an overload of `AbslFormatFlush()` for the
+// sink, usually by adding a ADL-based free function in the same namespace as
+// the sink:
+//
+//   void AbslFormatFlush(MySink* dest, absl::string_view part);
+//
+// where `dest` is the pointer passed to `absl::Format()`. The function should
+// append `part` to `dest`.
+//
 // FormatRawSink does not own the passed sink object. The passed object must
 // outlive the FormatRawSink.
 class FormatRawSink {
@@ -455,12 +464,13 @@
 // `absl::FormatRawSink` interface), using a format string and zero or more
 // additional arguments.
 //
-// By default, `std::string` and `std::ostream` are supported as destination
-// objects. If a `std::string` is used the formatted string is appended to it.
+// By default, `std::string`, `std::ostream`, and `absl::Cord` are supported as
+// destination objects. If a `std::string` is used the formatted string is
+// appended to it.
 //
-// `absl::Format()` is a generic version of `absl::StrFormat(), for custom
-// sinks. The format string, like format strings for `StrFormat()`, is checked
-// at compile-time.
+// `absl::Format()` is a generic version of `absl::StrAppendFormat()`, for
+// custom sinks. The format string, like format strings for `StrFormat()`, is
+// checked at compile-time.
 //
 // On failure, this function returns `false` and the state of the sink is
 // unspecified.

diff --git a/absl/strings/str_split_test.cc b/absl/strings/str_split_test.cc
index 67f62a7..fcd58d2 100644
--- a/absl/strings/str_split_test.cc
+++ b/absl/strings/str_split_test.cc

@@ -29,6 +29,8 @@
 #include "gtest/gtest.h"
 #include "absl/base/dynamic_annotations.h"  // for RunningOnValgrind
 #include "absl/base/macros.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/node_hash_map.h"
 #include "absl/strings/numbers.h"
 
 namespace {
@@ -421,6 +423,18 @@
   TestMapConversionOperator<std::multimap<std::string, std::string>>(splitter);
   TestMapConversionOperator<std::unordered_map<std::string, std::string>>(
       splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<absl::string_view, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::node_hash_map<std::string, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<absl::string_view, absl::string_view>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<absl::string_view, std::string>>(splitter);
+  TestMapConversionOperator<
+      absl::flat_hash_map<std::string, absl::string_view>>(splitter);
 
   // Tests conversion to std::pair
 

diff --git a/absl/synchronization/internal/mutex_nonprod.cc b/absl/synchronization/internal/mutex_nonprod.cc
index 4590b98..1732f83 100644
--- a/absl/synchronization/internal/mutex_nonprod.cc
+++ b/absl/synchronization/internal/mutex_nonprod.cc

@@ -32,6 +32,10 @@
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
+
+void SetMutexDeadlockDetectionMode(OnDeadlockCycle) {}
+void EnableMutexInvariantDebugging(bool) {}
+
 namespace synchronization_internal {
 
 namespace {

diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index a6e6d4c..ae83907 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h

@@ -101,7 +101,7 @@
 
   // How many periods to remain idle before releasing resources
 #ifndef THREAD_SANITIZER
-  static const int kIdlePeriods = 60;
+  static constexpr int kIdlePeriods = 60;
 #else
   // Memory consumption under ThreadSanitizer is a serious concern,
   // so we release resources sooner. The value of 1 leads to 1 to 2 second

diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 6ee5f23..8cda5a1 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc

@@ -1439,20 +1439,18 @@
 // may spin for a short while if the lock cannot be acquired immediately.
 static bool TryAcquireWithSpinning(std::atomic<intptr_t>* mu) {
   int c = mutex_globals.spinloop_iterations;
-  int result = -1;  // result of operation:  0=false, 1=true, -1=unknown
-
   do {  // do/while somewhat faster on AMD
     intptr_t v = mu->load(std::memory_order_relaxed);
-    if ((v & (kMuReader|kMuEvent)) != 0) {  // a reader or tracing -> give up
-      result = 0;
+    if ((v & (kMuReader|kMuEvent)) != 0) {
+      return false;  // a reader or tracing -> give up
     } else if (((v & kMuWriter) == 0) &&  // no holder -> try to acquire
                mu->compare_exchange_strong(v, kMuWriter | v,
                                            std::memory_order_acquire,
                                            std::memory_order_relaxed)) {
-      result = 1;
+      return true;
     }
-  } while (result == -1 && --c > 0);
-  return result == 1;
+  } while (--c > 0);
+  return false;
 }
 
 ABSL_XRAY_LOG_ARGS(1) void Mutex::Lock() {

diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index 8c70c4c..961ff52 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h

@@ -331,17 +331,16 @@
   // Mutex::AwaitWithTimeout()
   // Mutex::AwaitWithDeadline()
   //
-  // If `cond` is initially true, do nothing, or act as though `cond` is
-  // initially false.
-  //
-  // If `cond` is initially false, unlock this `Mutex` and block until
-  // simultaneously:
+  // Unlocks this `Mutex` and blocks until simultaneously:
   //   - either `cond` is true or the {timeout has expired, deadline has passed}
   //     and
   //   - this `Mutex` can be reacquired,
   // then reacquire this `Mutex` in the same mode in which it was previously
   // held, returning `true` iff `cond` is `true` on return.
   //
+  // If the condition is initially `true`, the implementation *may* skip the
+  // release/re-acquire step and return immediately.
+  //
   // Deadlines in the past are equivalent to an immediate deadline.
   // Negative timeouts are equivalent to a zero timeout.
   //

diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index 1353fa0..f018255 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc

@@ -732,9 +732,9 @@
 // Note: unit.prec is limited to double's digits10 value (typically 15) so it
 // always fits in buf[].
 void AppendNumberUnit(std::string* out, double n, DisplayUnit unit) {
-  const int buf_size = std::numeric_limits<double>::digits10;
-  const int prec = std::min(buf_size, unit.prec);
-  char buf[buf_size];  // also large enough to hold integer part
+  constexpr int kBufferSize = std::numeric_limits<double>::digits10;
+  const int prec = std::min(kBufferSize, unit.prec);
+  char buf[kBufferSize];  // also large enough to hold integer part
   char* ep = buf + sizeof(buf);
   double d = 0;
   int64_t frac_part = Round(std::modf(n, &d) * unit.pow10);

diff --git a/absl/time/time.h b/absl/time/time.h
index 33a4a63..152f3ff 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h

@@ -1348,8 +1348,8 @@
 // it's positive and can be converted to int64_t without risk of UB.
 inline Duration MakePosDoubleDuration(double n) {
   const int64_t int_secs = static_cast<int64_t>(n);
-  const uint32_t ticks =
-      static_cast<uint32_t>((n - int_secs) * kTicksPerSecond + 0.5);
+  const uint32_t ticks = static_cast<uint32_t>(
+      (n - static_cast<double>(int_secs)) * kTicksPerSecond + 0.5);
   return ticks < kTicksPerSecond
              ? MakeDuration(int_secs, ticks)
              : MakeDuration(int_secs + 1, ticks - kTicksPerSecond);

diff --git a/absl/types/BUILD.bazel b/absl/types/BUILD.bazel
index f2ea9f3..de71c73 100644
--- a/absl/types/BUILD.bazel
+++ b/absl/types/BUILD.bazel

@@ -35,6 +35,7 @@
         ":bad_any_cast",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:fast_type_id",
         "//absl/meta:type_traits",
         "//absl/utility",
     ],
@@ -215,11 +216,15 @@
         "internal/conformance_aliases.h",
         "internal/conformance_archetype.h",
         "internal/conformance_profile.h",
+        "internal/conformance_testing.h",
+        "internal/conformance_testing_helpers.h",
+        "internal/parentheses.h",
+        "internal/transform_args.h",
     ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        "//absl/debugging:demangle_internal",
+        "//absl/algorithm:container",
         "//absl/meta:type_traits",
         "//absl/strings",
         "//absl/utility",

diff --git a/absl/types/CMakeLists.txt b/absl/types/CMakeLists.txt
index c7c8825..0dc0d2c 100644
--- a/absl/types/CMakeLists.txt
+++ b/absl/types/CMakeLists.txt

@@ -24,6 +24,7 @@
     absl::bad_any_cast
     absl::config
     absl::core_headers
+    absl::fast_type_id
     absl::type_traits
     absl::utility
   PUBLIC
@@ -245,9 +246,14 @@
     "internal/conformance_aliases.h"
     "internal/conformance_archetype.h"
     "internal/conformance_profile.h"
+    "internal/conformance_testing.h"
+    "internal/conformance_testing_helpers.h"
+    "internal/parentheses.h"
+    "internal/transform_args.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::algorithm
     absl::debugging
     absl::type_traits
     absl::strings
@@ -281,6 +287,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::conformance_testing
+    absl::type_traits
     gmock_main
 )
 

diff --git a/absl/types/any.h b/absl/types/any.h
index 16bda79..7eed519 100644
--- a/absl/types/any.h
+++ b/absl/types/any.h

@@ -80,6 +80,7 @@
 #include <typeinfo>
 #include <utility>
 
+#include "absl/base/internal/fast_type_id.h"
 #include "absl/base/macros.h"
 #include "absl/meta/type_traits.h"
 #include "absl/types/bad_any_cast.h"
@@ -95,26 +96,6 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-namespace any_internal {
-
-template <typename Type>
-struct TypeTag {
-  constexpr static char dummy_var = 0;
-};
-
-template <typename Type>
-constexpr char TypeTag<Type>::dummy_var;
-
-// FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
-// passed in type. These are meant to be good match for keys into maps or
-// straight up comparisons.
-template<typename Type>
-constexpr inline const void* FastTypeId() {
-  return &TypeTag<Type>::dummy_var;
-}
-
-}  // namespace any_internal
-
 class any;
 
 // swap()
@@ -423,11 +404,11 @@
     using NormalizedType =
         typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
-    return any_internal::FastTypeId<NormalizedType>();
+    return base_internal::FastTypeId<NormalizedType>();
   }
 
   const void* GetObjTypeId() const {
-    return obj_ ? obj_->ObjTypeId() : any_internal::FastTypeId<void>();
+    return obj_ ? obj_->ObjTypeId() : base_internal::FastTypeId<void>();
   }
 
   // `absl::any` nonmember functions //

diff --git a/absl/types/internal/conformance_profile.h b/absl/types/internal/conformance_profile.h
index e62004f..cf64ff4 100644
--- a/absl/types/internal/conformance_profile.h
+++ b/absl/types/internal/conformance_profile.h

@@ -36,10 +36,19 @@
 #ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_
 #define ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_
 
+#include <set>
 #include <type_traits>
 #include <utility>
+#include <vector>
 
+#include "gtest/gtest.h"
+#include "absl/algorithm/container.h"
 #include "absl/meta/type_traits.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/internal/conformance_testing_helpers.h"
+#include "absl/utility/utility.h"
 
 // TODO(calabrese) Add support for extending profiles.
 
@@ -47,6 +56,187 @@
 ABSL_NAMESPACE_BEGIN
 namespace types_internal {
 
+// Converts an enum to its underlying integral value.
+template <typename Enum>
+constexpr absl::underlying_type_t<Enum> UnderlyingValue(Enum value) {
+  return static_cast<absl::underlying_type_t<Enum>>(value);
+}
+
+// A tag type used in place of a matcher when checking that an assertion result
+// does not actually contain any errors.
+struct NoError {};
+
+// -----------------------------------------------------------------------------
+// ConformanceErrors
+// -----------------------------------------------------------------------------
+class ConformanceErrors {
+ public:
+  // Setup the error reporting mechanism by seeding it with the name of the type
+  // that is being tested.
+  explicit ConformanceErrors(std::string type_name)
+      : assertion_result_(false), type_name_(std::move(type_name)) {
+    assertion_result_ << "\n\n"
+                         "Assuming the following type alias:\n"
+                         "\n"
+                         "  using _T = "
+                      << type_name_ << ";\n\n";
+    outputDivider();
+  }
+
+  // Adds the test name to the list of successfully run tests iff it was not
+  // previously reported as failing. This behavior is useful for tests that
+  // have multiple parts, where failures and successes are reported individually
+  // with the same test name.
+  void addTestSuccess(absl::string_view test_name) {
+    auto normalized_test_name = absl::AsciiStrToLower(test_name);
+
+    // If the test is already reported as failing, do not add it to the list of
+    // successes.
+    if (test_failures_.find(normalized_test_name) == test_failures_.end()) {
+      test_successes_.insert(std::move(normalized_test_name));
+    }
+  }
+
+  // Streams a single error description into the internal buffer (a visual
+  // divider is automatically inserted after the error so that multiple errors
+  // are visibly distinct).
+  //
+  // This function increases the error count by 1.
+  //
+  // TODO(calabrese) Determine desired behavior when if this function throws.
+  template <class... P>
+  void addTestFailure(absl::string_view test_name, const P&... args) {
+    // Output a message related to the test failure.
+    assertion_result_ << "\n\n"
+                         "Failed test: "
+                      << test_name << "\n\n";
+    addTestFailureImpl(args...);
+    assertion_result_ << "\n\n";
+    outputDivider();
+
+    auto normalized_test_name = absl::AsciiStrToLower(test_name);
+
+    // If previous parts of this test succeeded, remove it from that set.
+    test_successes_.erase(normalized_test_name);
+
+    // Add the test name to the list of failed tests.
+    test_failures_.insert(std::move(normalized_test_name));
+
+    has_error_ = true;
+  }
+
+  // Convert this object into a testing::AssertionResult instance such that it
+  // can be used with gtest.
+  ::testing::AssertionResult assertionResult() const {
+    return has_error_ ? assertion_result_ : ::testing::AssertionSuccess();
+  }
+
+  // Convert this object into a testing::AssertionResult instance such that it
+  // can be used with gtest. This overload expects errors, using the specified
+  // matcher.
+  ::testing::AssertionResult expectFailedTests(
+      const std::set<std::string>& test_names) const {
+    // Since we are expecting nonconformance, output an error message when the
+    // type actually conformed to the specified profile.
+    if (!has_error_) {
+      return ::testing::AssertionFailure()
+             << "Unexpected conformance of type:\n"
+                "    "
+             << type_name_ << "\n\n";
+    }
+
+    // Get a list of all expected failures that did not actually fail
+    // (or that were not run).
+    std::vector<std::string> nonfailing_tests;
+    absl::c_set_difference(test_names, test_failures_,
+                           std::back_inserter(nonfailing_tests));
+
+    // Get a list of all "expected failures" that were never actually run.
+    std::vector<std::string> unrun_tests;
+    absl::c_set_difference(nonfailing_tests, test_successes_,
+                           std::back_inserter(unrun_tests));
+
+    // Report when the user specified tests that were not run.
+    if (!unrun_tests.empty()) {
+      const bool tests_were_run =
+          !(test_failures_.empty() && test_successes_.empty());
+
+      // Prepare an assertion result used in the case that tests pass that were
+      // expected to fail.
+      ::testing::AssertionResult result = ::testing::AssertionFailure();
+      result << "When testing type:\n    " << type_name_
+             << "\n\nThe following tests were expected to fail but were not "
+                "run";
+
+      if (tests_were_run) result << " (was the test name spelled correctly?)";
+
+      result << ":\n\n";
+
+      // List all of the tests that unexpectedly passed.
+      for (const auto& test_name : unrun_tests) {
+        result << "    " << test_name << "\n";
+      }
+
+      if (!tests_were_run) result << "\nNo tests were run.";
+
+      if (!test_failures_.empty()) {
+        // List test failures
+        result << "\nThe tests that were run and failed are:\n\n";
+        for (const auto& test_name : test_failures_) {
+          result << "    " << test_name << "\n";
+        }
+      }
+
+      if (!test_successes_.empty()) {
+        // List test successes
+        result << "\nThe tests that were run and succeeded are:\n\n";
+        for (const auto& test_name : test_successes_) {
+          result << "    " << test_name << "\n";
+        }
+      }
+
+      return result;
+    }
+
+    // If some tests passed when they were expected to fail, alert the caller.
+    if (nonfailing_tests.empty()) return ::testing::AssertionSuccess();
+
+    // Prepare an assertion result used in the case that tests pass that were
+    // expected to fail.
+    ::testing::AssertionResult unexpected_successes =
+        ::testing::AssertionFailure();
+    unexpected_successes << "When testing type:\n    " << type_name_
+                         << "\n\nThe following tests passed when they were "
+                            "expected to fail:\n\n";
+
+    // List all of the tests that unexpectedly passed.
+    for (const auto& test_name : nonfailing_tests) {
+      unexpected_successes << "    " << test_name << "\n";
+    }
+
+    return unexpected_successes;
+  }
+
+ private:
+  void outputDivider() {
+    assertion_result_ << "========================================";
+  }
+
+  void addTestFailureImpl() {}
+
+  template <class H, class... T>
+  void addTestFailureImpl(const H& head, const T&... tail) {
+    assertion_result_ << head;
+    addTestFailureImpl(tail...);
+  }
+
+  ::testing::AssertionResult assertion_result_;
+  std::set<std::string> test_failures_;
+  std::set<std::string> test_successes_;
+  std::string type_name_;
+  bool has_error_ = false;
+};
+
 template <class T, class /*Enabler*/ = void>
 struct PropertiesOfImpl {};
 
@@ -70,31 +260,100 @@
 // standard trait names, which is useful since it allows us to match up each
 // enum name with a corresponding trait name in macro definitions.
 
-enum class function_kind { maybe, yes, nothrow, trivial };
+// An enum that describes the various expectations on an operations existence.
+enum class function_support { maybe, yes, nothrow, trivial };
 
-#define ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(name) \
-  enum class name { maybe, yes, nothrow, trivial }
+constexpr const char* PessimisticPropertyDescription(function_support v) {
+  return v == function_support::maybe
+             ? "no"
+             : v == function_support::yes
+                   ? "yes, potentially throwing"
+                   : v == function_support::nothrow ? "yes, nothrow"
+                                                    : "yes, trivial";
+}
 
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(default_constructible);
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(move_constructible);
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(copy_constructible);
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(move_assignable);
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(copy_assignable);
-ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(destructible);
+// Return a string that describes the kind of property support that was
+// expected.
+inline std::string ExpectedFunctionKindList(function_support min,
+                                            function_support max) {
+  if (min == max) {
+    std::string result =
+        absl::StrCat("Expected:\n  ",
+                     PessimisticPropertyDescription(
+                         static_cast<function_support>(UnderlyingValue(min))),
+                     "\n");
+    return result;
+  }
+
+  std::string result = "Expected one of:\n";
+  for (auto curr_support = UnderlyingValue(min);
+       curr_support <= UnderlyingValue(max); ++curr_support) {
+    absl::StrAppend(&result, "  ",
+                    PessimisticPropertyDescription(
+                        static_cast<function_support>(curr_support)),
+                    "\n");
+  }
+
+  return result;
+}
+
+template <class Enum>
+void ExpectModelOfImpl(ConformanceErrors* errors, Enum min_support,
+                       Enum max_support, Enum kind) {
+  const auto kind_value = UnderlyingValue(kind);
+  const auto min_support_value = UnderlyingValue(min_support);
+  const auto max_support_value = UnderlyingValue(max_support);
+
+  if (!(kind_value >= min_support_value && kind_value <= max_support_value)) {
+    errors->addTestFailure(
+        PropertyName(kind), "**Failed property expectation**\n\n",
+        ExpectedFunctionKindList(
+            static_cast<function_support>(min_support_value),
+            static_cast<function_support>(max_support_value)),
+        '\n', "Actual:\n  ",
+        PessimisticPropertyDescription(
+            static_cast<function_support>(kind_value)));
+  } else {
+    errors->addTestSuccess(PropertyName(kind));
+  }
+}
+
+#define ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(description, name) \
+  enum class name { maybe, yes, nothrow, trivial };                   \
+                                                                      \
+  constexpr const char* PropertyName(name v) { return description; }  \
+  static_assert(true, "")  // Force a semicolon when using this macro.
+
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for default construction",
+                                           default_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for move construction",
+                                           move_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for copy construction",
+                                           copy_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for move assignment",
+                                           move_assignable);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for copy assignment",
+                                           copy_assignable);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM("support for destruction",
+                                           destructible);
 
 #undef ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM
 
-#define ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(name) \
-  enum class name { maybe, yes, nothrow }
+#define ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(description, name)     \
+  enum class name { maybe, yes, nothrow };                           \
+                                                                     \
+  constexpr const char* PropertyName(name v) { return description; } \
+  static_assert(true, "")  // Force a semicolon when using this macro.
 
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(equality_comparable);
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(inequality_comparable);
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(less_than_comparable);
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(less_equal_comparable);
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(greater_equal_comparable);
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(greater_than_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for ==", equality_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for !=", inequality_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for <", less_than_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for <=", less_equal_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for >=",
+                                      greater_equal_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for >", greater_than_comparable);
 
-ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(swappable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM("support for swap", swappable);
 
 #undef ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM
 
@@ -104,6 +363,184 @@
   return "support for std::hash";
 }
 
+template <class T>
+using AlwaysFalse = std::false_type;
+
+#define ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(name, property)   \
+  template <class T>                                                        \
+  constexpr property property##_support_of() {                              \
+    return std::is_##property<T>::value                                     \
+               ? std::is_nothrow_##property<T>::value                       \
+                     ? absl::is_trivially_##property<T>::value              \
+                           ? property::trivial                              \
+                           : property::nothrow                              \
+                     : property::yes                                        \
+               : property::maybe;                                           \
+  }                                                                         \
+                                                                            \
+  template <class T, class MinProf, class MaxProf>                          \
+  void ExpectModelOf##name(ConformanceErrors* errors) {                     \
+    (ExpectModelOfImpl)(errors, PropertiesOfT<MinProf>::property##_support, \
+                        PropertiesOfT<MaxProf>::property##_support,         \
+                        property##_support_of<T>());                        \
+  }
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(DefaultConstructible,
+                                                  default_constructible);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(MoveConstructible,
+                                                  move_constructible);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(CopyConstructible,
+                                                  copy_constructible);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(MoveAssignable,
+                                                  move_assignable);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(CopyAssignable,
+                                                  copy_assignable);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER(Destructible, destructible);
+
+#undef ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_SPECIAL_MEMBER
+
+void BoolFunction(bool) noexcept;
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction for checking if an operation exists through SFINAE.
+//
+// `T` is the type to test and Op is an alias containing the expression to test.
+template <class T, template <class...> class Op, class = void>
+struct IsOpableImpl : std::false_type {};
+
+template <class T, template <class...> class Op>
+struct IsOpableImpl<T, Op, absl::void_t<Op<T>>> : std::true_type {};
+
+template <template <class...> class Op>
+struct IsOpable {
+  template <class T>
+  using apply = typename IsOpableImpl<T, Op>::type;
+};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction for checking if an operation exists and is also noexcept
+// through SFINAE and the noexcept operator.
+///
+// `T` is the type to test and Op is an alias containing the expression to test.
+template <class T, template <class...> class Op, class = void>
+struct IsNothrowOpableImpl : std::false_type {};
+
+template <class T, template <class...> class Op>
+struct IsNothrowOpableImpl<T, Op, absl::enable_if_t<Op<T>::value>>
+    : std::true_type {};
+
+template <template <class...> class Op>
+struct IsNothrowOpable {
+  template <class T>
+  using apply = typename IsNothrowOpableImpl<T, Op>::type;
+};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A macro that produces the necessary function for reporting what kind of
+// support a specific comparison operation has and a function for reporting an
+// error if a given type's support for that operation does not meet the expected
+// requirements.
+#define ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(name, property, op)      \
+  template <class T,                                                           \
+            class Result = std::integral_constant<                             \
+                bool, noexcept((BoolFunction)(std::declval<const T&>() op      \
+                                                  std::declval<const T&>()))>> \
+  using name = Result;                                                         \
+                                                                               \
+  template <class T>                                                           \
+  constexpr property property##_support_of() {                                 \
+    return IsOpable<name>::apply<T>::value                                     \
+               ? IsNothrowOpable<name>::apply<T>::value ? property::nothrow    \
+                                                        : property::yes        \
+               : property::maybe;                                              \
+  }                                                                            \
+                                                                               \
+  template <class T, class MinProf, class MaxProf>                             \
+  void ExpectModelOf##name(ConformanceErrors* errors) {                        \
+    (ExpectModelOfImpl)(errors, PropertiesOfT<MinProf>::property##_support,    \
+                        PropertiesOfT<MaxProf>::property##_support,            \
+                        property##_support_of<T>());                           \
+  }
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Generate the necessary support-checking and error reporting functions for
+// each of the comparison operators.
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(EqualityComparable,
+                                              equality_comparable, ==);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(InequalityComparable,
+                                              inequality_comparable, !=);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(LessThanComparable,
+                                              less_than_comparable, <);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(LessEqualComparable,
+                                              less_equal_comparable, <=);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(GreaterEqualComparable,
+                                              greater_equal_comparable, >=);
+
+ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON(GreaterThanComparable,
+                                              greater_than_comparable, >);
+
+#undef ABSL_INTERNAL_PESSIMISTIC_MODEL_OF_COMPARISON
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// The necessary support-checking and error-reporting functions for swap.
+template <class T>
+constexpr swappable swappable_support_of() {
+  return type_traits_internal::IsSwappable<T>::value
+             ? type_traits_internal::IsNothrowSwappable<T>::value
+                   ? swappable::nothrow
+                   : swappable::yes
+             : swappable::maybe;
+}
+
+template <class T, class MinProf, class MaxProf>
+void ExpectModelOfSwappable(ConformanceErrors* errors) {
+  (ExpectModelOfImpl)(errors, PropertiesOfT<MinProf>::swappable_support,
+                      PropertiesOfT<MaxProf>::swappable_support,
+                      swappable_support_of<T>());
+}
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// The necessary support-checking and error-reporting functions for std::hash.
+template <class T>
+constexpr hashable hashable_support_of() {
+  return type_traits_internal::IsHashable<T>::value ? hashable::yes
+                                                    : hashable::maybe;
+}
+
+template <class T, class MinProf, class MaxProf>
+void ExpectModelOfHashable(ConformanceErrors* errors) {
+  (ExpectModelOfImpl)(errors, PropertiesOfT<MinProf>::hashable_support,
+                      PropertiesOfT<MaxProf>::hashable_support,
+                      hashable_support_of<T>());
+}
+//
+////////////////////////////////////////////////////////////////////////////////
+
 template <
     default_constructible DefaultConstructibleValue =
         default_constructible::maybe,
@@ -216,6 +653,45 @@
       HashableValue != hashable::maybe;
 };
 
+////////////////////////////////////////////////////////////////////////////////
+//
+// Compliant SFINAE-friendliness is not always present on the standard library
+// implementations that we support. This helper-struct (and associated enum) is
+// used as a means to conditionally check the hashability support of a type.
+enum class CheckHashability { no, yes };
+
+template <class T, CheckHashability ShouldCheckHashability>
+struct conservative_hashable_support_of;
+
+template <class T>
+struct conservative_hashable_support_of<T, CheckHashability::no> {
+  static constexpr hashable Invoke() { return hashable::maybe; }
+};
+
+template <class T>
+struct conservative_hashable_support_of<T, CheckHashability::yes> {
+  static constexpr hashable Invoke() { return hashable_support_of<T>(); }
+};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// The ConformanceProfile that is expected based on introspection into the type
+// by way of trait checks.
+template <class T, CheckHashability ShouldCheckHashability>
+struct SyntacticConformanceProfileOf {
+  using properties = ConformanceProfile<
+      default_constructible_support_of<T>(), move_constructible_support_of<T>(),
+      copy_constructible_support_of<T>(), move_assignable_support_of<T>(),
+      copy_assignable_support_of<T>(), destructible_support_of<T>(),
+      equality_comparable_support_of<T>(),
+      inequality_comparable_support_of<T>(),
+      less_than_comparable_support_of<T>(),
+      less_equal_comparable_support_of<T>(),
+      greater_equal_comparable_support_of<T>(),
+      greater_than_comparable_support_of<T>(), swappable_support_of<T>(),
+      conservative_hashable_support_of<T, ShouldCheckHashability>::Invoke()>;
+};
+
 #define ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL(type, name)     \
   template <default_constructible DefaultConstructibleValue,                   \
             move_constructible MoveConstructibleValue,                         \
@@ -261,12 +737,80 @@
 #undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF
 #undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL
 
-// Converts an enum to its underlying integral value.
-template <class Enum>
-constexpr absl::underlying_type_t<Enum> UnderlyingValue(Enum value) {
-  return static_cast<absl::underlying_type_t<Enum>>(value);
+// Retrieve the enum with the minimum underlying value.
+// Note: std::min is not constexpr in C++11, which is why this is necessary.
+template <class H>
+constexpr H MinEnum(H head) {
+  return head;
 }
 
+template <class H, class N, class... T>
+constexpr H MinEnum(H head, N next, T... tail) {
+  return (UnderlyingValue)(head) < (UnderlyingValue)(next)
+             ? (MinEnum)(head, tail...)
+             : (MinEnum)(next, tail...);
+}
+
+template <class... Profs>
+struct MinimalProfiles {
+  static constexpr default_constructible
+      default_constructible_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::default_constructible_support...);
+
+  static constexpr move_constructible move_constructible_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::move_constructible_support...);
+
+  static constexpr copy_constructible copy_constructible_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::copy_constructible_support...);
+
+  static constexpr move_assignable move_assignable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::move_assignable_support...);
+
+  static constexpr copy_assignable copy_assignable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::copy_assignable_support...);
+
+  static constexpr destructible destructible_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::destructible_support...);
+
+  static constexpr equality_comparable equality_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::equality_comparable_support...);
+
+  static constexpr inequality_comparable
+      inequality_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::inequality_comparable_support...);
+
+  static constexpr less_than_comparable
+      less_than_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::less_than_comparable_support...);
+
+  static constexpr less_equal_comparable
+      less_equal_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::less_equal_comparable_support...);
+
+  static constexpr greater_equal_comparable
+      greater_equal_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::greater_equal_comparable_support...);
+
+  static constexpr greater_than_comparable
+      greater_than_comparable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::greater_than_comparable_support...);
+
+  static constexpr swappable swappable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::swappable_support...);
+
+  static constexpr hashable hashable_support =  // NOLINT
+      (MinEnum)(PropertiesOfT<Profs>::hashable_support...);
+
+  using properties = ConformanceProfile<
+      default_constructible_support, move_constructible_support,
+      copy_constructible_support, move_assignable_support,
+      copy_assignable_support, destructible_support,
+      equality_comparable_support, inequality_comparable_support,
+      less_than_comparable_support, less_equal_comparable_support,
+      greater_equal_comparable_support, greater_than_comparable_support,
+      swappable_support, hashable_support>;
+};
+
 // Retrieve the enum with the greatest underlying value.
 // Note: std::max is not constexpr in C++11, which is why this is necessary.
 template <class H>
@@ -369,6 +913,17 @@
 template <class T>
 struct IsProfile : IsProfileImpl<T>::type {};
 
+// A tag that describes which set of properties we will check when the user
+// requires a strict match in conformance (as opposed to a loose match which
+// allows more-refined support of any given operation).
+//
+// Currently only the RegularityDomain exists and it includes all operations
+// that the conformance testing suite knows about. The intent is that if the
+// suite is expanded to support extension, such as for checking conformance of
+// concepts like Iterators or Containers, additional corresponding domains can
+// be created.
+struct RegularityDomain {};
+
 }  // namespace types_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

diff --git a/absl/types/internal/conformance_testing.h b/absl/types/internal/conformance_testing.h
new file mode 100644
index 0000000..487b0f7
--- /dev/null
+++ b/absl/types/internal/conformance_testing.h

@@ -0,0 +1,1386 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// conformance_testing.h
+// -----------------------------------------------------------------------------
+//
+
+#ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_H_
+#define ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_H_
+
+////////////////////////////////////////////////////////////////////////////////
+//                                                                            //
+// Many templates in this file take a `T` and a `Prof` type as explicit       //
+// template arguments. These are a type to be checked and a                   //
+// "Regularity Profile" that describes what operations that type `T` is       //
+// expected to support. See "regularity_profiles.h" for more details          //
+// regarding Regularity Profiles.                                             //
+//                                                                            //
+////////////////////////////////////////////////////////////////////////////////
+
+#include <cstddef>
+#include <set>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/ascii.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/internal/conformance_aliases.h"
+#include "absl/types/internal/conformance_archetype.h"
+#include "absl/types/internal/conformance_profile.h"
+#include "absl/types/internal/conformance_testing_helpers.h"
+#include "absl/types/internal/parentheses.h"
+#include "absl/types/internal/transform_args.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+
+// Returns true if the compiler incorrectly greedily instantiates constexpr
+// templates in any unevaluated context.
+constexpr bool constexpr_instantiation_when_unevaluated() {
+#if defined(__apple_build_version__)  // TODO(calabrese) Make more specific
+  return true;
+#elif defined(__clang__)
+  return __clang_major__ < 4;
+#elif defined(__GNUC__)
+  // TODO(calabrese) Figure out why gcc 7 fails (seems like a different bug)
+  return __GNUC__ < 5 || (__GNUC__ == 5 && __GNUC_MINOR__ < 2) || __GNUC__ >= 7;
+#else
+  return false;
+#endif
+}
+
+// Returns true if the standard library being used incorrectly produces an error
+// when instantiating the definition of a poisoned std::hash specialization.
+constexpr bool poisoned_hash_fails_instantiation() {
+#if defined(_MSC_VER) && !defined(_LIBCPP_VERSION)
+  return _MSC_VER < 1914;
+#else
+  return false;
+#endif
+}
+
+template <class Fun>
+struct GeneratorType {
+  decltype(std::declval<const Fun&>()()) operator()() const
+      noexcept(noexcept(std::declval<const Fun&>()())) {
+    return fun();
+  }
+
+  Fun fun;
+  const char* description;
+};
+
+// A "make" function for the GeneratorType template that deduces the function
+// object type.
+template <class Fun,
+          absl::enable_if_t<IsNullaryCallable<Fun>::value>** = nullptr>
+GeneratorType<Fun> Generator(Fun fun, const char* description) {
+  return GeneratorType<Fun>{absl::move(fun), description};
+}
+
+// A type that contains a set of nullary function objects that each return an
+// instance of the same type and value (though possibly different
+// representations, such as +0 and -0 or two vectors with the same elements but
+// with different capacities).
+template <class... Funs>
+struct EquivalenceClassType {
+  std::tuple<GeneratorType<Funs>...> generators;
+};
+
+// A "make" function for the EquivalenceClassType template that deduces the
+// function object types and is constrained such that a user can only pass in
+// function objects that all have the same return type.
+template <class... Funs, absl::enable_if_t<AreGeneratorsWithTheSameReturnType<
+                             Funs...>::value>** = nullptr>
+EquivalenceClassType<Funs...> EquivalenceClass(GeneratorType<Funs>... funs) {
+  return {std::make_tuple(absl::move(funs)...)};
+}
+
+// A type that contains an ordered series of EquivalenceClassTypes, from
+// smallest value to largest value.
+template <class... EqClasses>
+struct OrderedEquivalenceClasses {
+  std::tuple<EqClasses...> eq_classes;
+};
+
+// An object containing the parts of a given (name, initialization expression),
+// and is capable of generating a string that describes the given.
+struct GivenDeclaration {
+  std::string outputDeclaration(std::size_t width) const {
+    const std::size_t indent_size = 2;
+    std::string result = absl::StrCat("  ", name);
+
+    if (!expression.empty()) {
+      // Indent
+      result.resize(indent_size + width, ' ');
+      absl::StrAppend(&result, " = ", expression, ";\n");
+    } else {
+      absl::StrAppend(&result, ";\n");
+    }
+
+    return result;
+  }
+
+  std::string name;
+  std::string expression;
+};
+
+// Produce a string that contains all of the givens of an error report.
+template <class... Decls>
+std::string PrepareGivenContext(const Decls&... decls) {
+  const std::size_t width = (std::max)({decls.name.size()...});
+  return absl::StrCat("Given:\n", decls.outputDeclaration(width)..., "\n");
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Function objects that perform a check for each comparison operator         //
+////////////////////////////////////////////////////////////////////////////////
+
+#define ABSL_INTERNAL_EXPECT_OP(name, op)                                   \
+  struct Expect##name {                                                     \
+    template <class T>                                                      \
+    void operator()(absl::string_view test_name, absl::string_view context, \
+                    const T& lhs, const T& rhs, absl::string_view lhs_name, \
+                    absl::string_view rhs_name) const {                     \
+      if (!static_cast<bool>(lhs op rhs)) {                                 \
+        errors->addTestFailure(                                             \
+            test_name, absl::StrCat(context,                                \
+                                    "**Unexpected comparison result**\n"    \
+                                    "\n"                                    \
+                                    "Expression:\n"                         \
+                                    "  ",                                   \
+                                    lhs_name, " " #op " ", rhs_name,        \
+                                    "\n"                                    \
+                                    "\n"                                    \
+                                    "Expected: true\n"                      \
+                                    "  Actual: false"));                    \
+      } else {                                                              \
+        errors->addTestSuccess(test_name);                                  \
+      }                                                                     \
+    }                                                                       \
+                                                                            \
+    ConformanceErrors* errors;                                              \
+  };                                                                        \
+                                                                            \
+  struct ExpectNot##name {                                                  \
+    template <class T>                                                      \
+    void operator()(absl::string_view test_name, absl::string_view context, \
+                    const T& lhs, const T& rhs, absl::string_view lhs_name, \
+                    absl::string_view rhs_name) const {                     \
+      if (lhs op rhs) {                                                     \
+        errors->addTestFailure(                                             \
+            test_name, absl::StrCat(context,                                \
+                                    "**Unexpected comparison result**\n"    \
+                                    "\n"                                    \
+                                    "Expression:\n"                         \
+                                    "  ",                                   \
+                                    lhs_name, " " #op " ", rhs_name,        \
+                                    "\n"                                    \
+                                    "\n"                                    \
+                                    "Expected: false\n"                     \
+                                    "  Actual: true"));                     \
+      } else {                                                              \
+        errors->addTestSuccess(test_name);                                  \
+      }                                                                     \
+    }                                                                       \
+                                                                            \
+    ConformanceErrors* errors;                                              \
+  }
+
+ABSL_INTERNAL_EXPECT_OP(Eq, ==);
+ABSL_INTERNAL_EXPECT_OP(Ne, !=);
+ABSL_INTERNAL_EXPECT_OP(Lt, <);
+ABSL_INTERNAL_EXPECT_OP(Le, <=);
+ABSL_INTERNAL_EXPECT_OP(Ge, >=);
+ABSL_INTERNAL_EXPECT_OP(Gt, >);
+
+#undef ABSL_INTERNAL_EXPECT_OP
+
+// A function object that verifies that two objects hash to the same value by
+// way of the std::hash specialization.
+struct ExpectSameHash {
+  template <class T>
+  void operator()(absl::string_view test_name, absl::string_view context,
+                  const T& lhs, const T& rhs, absl::string_view lhs_name,
+                  absl::string_view rhs_name) const {
+    if (std::hash<T>()(lhs) != std::hash<T>()(rhs)) {
+      errors->addTestFailure(
+          test_name, absl::StrCat(context,
+                                  "**Unexpected hash result**\n"
+                                  "\n"
+                                  "Expression:\n"
+                                  "  std::hash<T>()(",
+                                  lhs_name, ") == std::hash<T>()(", rhs_name,
+                                  ")\n"
+                                  "\n"
+                                  "Expected: true\n"
+                                  "  Actual: false"));
+    } else {
+      errors->addTestSuccess(test_name);
+    }
+  }
+
+  ConformanceErrors* errors;
+};
+
+// A function template that takes two objects and verifies that each comparison
+// operator behaves in a way that is consistent with equality. It has "OneWay"
+// in the name because the first argument will always be the left-hand operand
+// of the corresponding comparison operator and the second argument will
+// always be the right-hand operand. It will never switch that order.
+// At a higher level in the test suite, the one-way form is called once for each
+// of the two possible orders whenever lhs and rhs are not the same initializer.
+template <class T, class Prof>
+void ExpectOneWayEquality(ConformanceErrors* errors,
+                          absl::string_view test_name,
+                          absl::string_view context, const T& lhs, const T& rhs,
+                          absl::string_view lhs_name,
+                          absl::string_view rhs_name) {
+  If<PropertiesOfT<Prof>::is_equality_comparable>::Invoke(
+      ExpectEq{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_inequality_comparable>::Invoke(
+      ExpectNotNe{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_less_than_comparable>::Invoke(
+      ExpectNotLt{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_less_equal_comparable>::Invoke(
+      ExpectLe{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_greater_equal_comparable>::Invoke(
+      ExpectGe{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_greater_than_comparable>::Invoke(
+      ExpectNotGt{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+
+  If<PropertiesOfT<Prof>::is_hashable>::Invoke(
+      ExpectSameHash{errors}, test_name, context, lhs, rhs, lhs_name, rhs_name);
+}
+
+// A function template that takes two objects and verifies that each comparison
+// operator behaves in a way that is consistent with equality. This function
+// differs from ExpectOneWayEquality in that this will do checks with argument
+// order reversed in addition to in-order.
+template <class T, class Prof>
+void ExpectEquality(ConformanceErrors* errors, absl::string_view test_name,
+                    absl::string_view context, const T& lhs, const T& rhs,
+                    absl::string_view lhs_name, absl::string_view rhs_name) {
+  (ExpectOneWayEquality<T, Prof>)(errors, test_name, context, lhs, rhs,
+                                  lhs_name, rhs_name);
+  (ExpectOneWayEquality<T, Prof>)(errors, test_name, context, rhs, lhs,
+                                  rhs_name, lhs_name);
+}
+
+// Given a generator, makes sure that a generated value and a moved-from
+// generated value are equal.
+template <class T, class Prof>
+struct ExpectMoveConstructOneGenerator {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T object = generator();
+    const T moved_object = absl::move(generator());  // Force no elision.
+
+    (ExpectEquality<T, Prof>)(errors, "Move construction",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T object",
+                                                   generator.description},
+                                  GivenDeclaration{"const _T moved_object",
+                                                   std::string("std::move(") +
+                                                       generator.description +
+                                                       ")"}),
+                              object, moved_object, "object", "moved_object");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Given a generator, makes sure that a generated value and a copied-from
+// generated value are equal.
+template <class T, class Prof>
+struct ExpectCopyConstructOneGenerator {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T object = generator();
+    const T copied_object = static_cast<const T&>(generator());
+
+    (ExpectEquality<T, Prof>)(errors, "Copy construction",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T object",
+                                                   generator.description},
+                                  GivenDeclaration{
+                                      "const _T copied_object",
+                                      std::string("static_cast<const _T&>(") +
+                                          generator.description + ")"}),
+                              object, copied_object, "object", "copied_object");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Default-construct and do nothing before destruction.
+//
+// This is useful in exercising the codepath of default construction followed by
+// destruction, but does not explicitly test anything. An example of where this
+// might fail is a default destructor that default-initializes a scalar and a
+// destructor reads the value of that member. Sanitizers can catch this as long
+// as our test attempts to execute such a case.
+template <class T>
+struct ExpectDefaultConstructWithDestruct {
+  void operator()() const {
+    // Scoped so that destructor gets called before reporting success.
+    {
+      T object;
+      static_cast<void>(object);
+    }
+
+    errors->addTestSuccess("Default construction");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Check move-assign into a default-constructed object.
+template <class T, class Prof>
+struct ExpectDefaultConstructWithMoveAssign {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T source_of_truth = generator();
+    T object;
+    object = generator();
+
+    (ExpectEquality<T, Prof>)(errors, "Move assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T object",
+                                                   generator.description},
+                                  GivenDeclaration{"_T object", ""},
+                                  GivenDeclaration{"object",
+                                                   generator.description}),
+                              object, source_of_truth, "std::as_const(object)",
+                              "source_of_truth");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Check copy-assign into a default-constructed object.
+template <class T, class Prof>
+struct ExpectDefaultConstructWithCopyAssign {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T source_of_truth = generator();
+    T object;
+    object = static_cast<const T&>(generator());
+
+    (ExpectEquality<T, Prof>)(errors, "Copy assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T source_of_truth",
+                                                   generator.description},
+                                  GivenDeclaration{"_T object", ""},
+                                  GivenDeclaration{
+                                      "object",
+                                      std::string("static_cast<const _T&>(") +
+                                          generator.description + ")"}),
+                              object, source_of_truth, "std::as_const(object)",
+                              "source_of_truth");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Perform a self move-assign.
+template <class T, class Prof>
+struct ExpectSelfMoveAssign {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    T object = generator();
+    object = absl::move(object);
+
+    // NOTE: Self move-assign results in a valid-but-unspecified state.
+
+    (ExpectEquality<T, Prof>)(errors, "Move assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"_T object",
+                                                   generator.description},
+                                  GivenDeclaration{"object",
+                                                   "std::move(object)"}),
+                              object, object, "object", "object");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Perform a self copy-assign.
+template <class T, class Prof>
+struct ExpectSelfCopyAssign {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T source_of_truth = generator();
+    T object = generator();
+    const T& const_object = object;
+    object = const_object;
+
+    (ExpectEquality<T, Prof>)(errors, "Copy assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T source_of_truth",
+                                                   generator.description},
+                                  GivenDeclaration{"_T object",
+                                                   generator.description},
+                                  GivenDeclaration{"object",
+                                                   "std::as_const(object)"}),
+                              const_object, source_of_truth,
+                              "std::as_const(object)", "source_of_truth");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Perform a self-swap.
+template <class T, class Prof>
+struct ExpectSelfSwap {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T source_of_truth = generator();
+    T object = generator();
+
+    type_traits_internal::Swap(object, object);
+
+    std::string preliminary_info = absl::StrCat(
+        PrepareGivenContext(
+            GivenDeclaration{"const _T source_of_truth", generator.description},
+            GivenDeclaration{"_T object", generator.description}),
+        "After performing a self-swap:\n"
+        "  using std::swap;\n"
+        "  swap(object, object);\n"
+        "\n");
+
+    (ExpectEquality<T, Prof>)(errors, "Swap", std::move(preliminary_info),
+                              object, source_of_truth, "std::as_const(object)",
+                              "source_of_truth");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Perform each of the single-generator checks when necessary operations are
+// supported.
+template <class T, class Prof>
+struct ExpectSelfComparison {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    const T object = generator();
+    (ExpectOneWayEquality<T, Prof>)(errors, "Comparison",
+                                    PrepareGivenContext(GivenDeclaration{
+                                        "const _T object",
+                                        generator.description}),
+                                    object, object, "object", "object");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Perform each of the single-generator checks when necessary operations are
+// supported.
+template <class T, class Prof>
+struct ExpectConsistency {
+  template <class Fun>
+  void operator()(const Fun& generator) const {
+    If<PropertiesOfT<Prof>::is_move_constructible>::Invoke(
+        ExpectMoveConstructOneGenerator<T, Prof>{errors}, generator);
+
+    If<PropertiesOfT<Prof>::is_copy_constructible>::Invoke(
+        ExpectCopyConstructOneGenerator<T, Prof>{errors}, generator);
+
+    If<PropertiesOfT<Prof>::is_default_constructible &&
+       PropertiesOfT<Prof>::is_move_assignable>::
+        Invoke(ExpectDefaultConstructWithMoveAssign<T, Prof>{errors},
+               generator);
+
+    If<PropertiesOfT<Prof>::is_default_constructible &&
+       PropertiesOfT<Prof>::is_copy_assignable>::
+        Invoke(ExpectDefaultConstructWithCopyAssign<T, Prof>{errors},
+               generator);
+
+    If<PropertiesOfT<Prof>::is_move_assignable>::Invoke(
+        ExpectSelfMoveAssign<T, Prof>{errors}, generator);
+
+    If<PropertiesOfT<Prof>::is_copy_assignable>::Invoke(
+        ExpectSelfCopyAssign<T, Prof>{errors}, generator);
+
+    If<PropertiesOfT<Prof>::is_swappable>::Invoke(
+        ExpectSelfSwap<T, Prof>{errors}, generator);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Check move-assign with two different values.
+template <class T, class Prof>
+struct ExpectMoveAssign {
+  template <class Fun0, class Fun1>
+  void operator()(const Fun0& generator0, const Fun1& generator1) const {
+    const T source_of_truth1 = generator1();
+    T object = generator0();
+    object = generator1();
+
+    (ExpectEquality<T, Prof>)(errors, "Move assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T source_of_truth1",
+                                                   generator1.description},
+                                  GivenDeclaration{"_T object",
+                                                   generator0.description},
+                                  GivenDeclaration{"object",
+                                                   generator1.description}),
+                              object, source_of_truth1, "std::as_const(object)",
+                              "source_of_truth1");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Check copy-assign with two different values.
+template <class T, class Prof>
+struct ExpectCopyAssign {
+  template <class Fun0, class Fun1>
+  void operator()(const Fun0& generator0, const Fun1& generator1) const {
+    const T source_of_truth1 = generator1();
+    T object = generator0();
+    object = static_cast<const T&>(generator1());
+
+    (ExpectEquality<T, Prof>)(errors, "Copy assignment",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T source_of_truth1",
+                                                   generator1.description},
+                                  GivenDeclaration{"_T object",
+                                                   generator0.description},
+                                  GivenDeclaration{
+                                      "object",
+                                      std::string("static_cast<const _T&>(") +
+                                          generator1.description + ")"}),
+                              object, source_of_truth1, "std::as_const(object)",
+                              "source_of_truth1");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Check swap with two different values.
+template <class T, class Prof>
+struct ExpectSwap {
+  template <class Fun0, class Fun1>
+  void operator()(const Fun0& generator0, const Fun1& generator1) const {
+    const T source_of_truth0 = generator0();
+    const T source_of_truth1 = generator1();
+    T object0 = generator0();
+    T object1 = generator1();
+
+    type_traits_internal::Swap(object0, object1);
+
+    const std::string context =
+        PrepareGivenContext(
+            GivenDeclaration{"const _T source_of_truth0",
+                             generator0.description},
+            GivenDeclaration{"const _T source_of_truth1",
+                             generator1.description},
+            GivenDeclaration{"_T object0", generator0.description},
+            GivenDeclaration{"_T object1", generator1.description}) +
+        "After performing a swap:\n"
+        "  using std::swap;\n"
+        "  swap(object0, object1);\n"
+        "\n";
+
+    (ExpectEquality<T, Prof>)(errors, "Swap", context, object0,
+                              source_of_truth1, "std::as_const(object0)",
+                              "source_of_truth1");
+    (ExpectEquality<T, Prof>)(errors, "Swap", context, object1,
+                              source_of_truth0, "std::as_const(object1)",
+                              "source_of_truth0");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Validate that `generator0` and `generator1` produce values that are equal.
+template <class T, class Prof>
+struct ExpectEquivalenceClassComparison {
+  template <class Fun0, class Fun1>
+  void operator()(const Fun0& generator0, const Fun1& generator1) const {
+    const T object0 = generator0();
+    const T object1 = generator1();
+
+    (ExpectEquality<T, Prof>)(errors, "Comparison",
+                              PrepareGivenContext(
+                                  GivenDeclaration{"const _T object0",
+                                                   generator0.description},
+                                  GivenDeclaration{"const _T object1",
+                                                   generator1.description}),
+                              object0, object1, "object0", "object1");
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Validate that all objects in the same equivalence-class have the same value.
+template <class T, class Prof>
+struct ExpectEquivalenceClassConsistency {
+  template <class Fun0, class Fun1>
+  void operator()(const Fun0& generator0, const Fun1& generator1) const {
+    If<PropertiesOfT<Prof>::is_move_assignable>::Invoke(
+        ExpectMoveAssign<T, Prof>{errors}, generator0, generator1);
+
+    If<PropertiesOfT<Prof>::is_copy_assignable>::Invoke(
+        ExpectCopyAssign<T, Prof>{errors}, generator0, generator1);
+
+    If<PropertiesOfT<Prof>::is_swappable>::Invoke(ExpectSwap<T, Prof>{errors},
+                                                  generator0, generator1);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Given a "lesser" object and a "greater" object, perform every combination of
+// comparison operators supported for the type, expecting consistent results.
+template <class T, class Prof>
+void ExpectOrdered(ConformanceErrors* errors, absl::string_view context,
+                   const T& small, const T& big, absl::string_view small_name,
+                   absl::string_view big_name) {
+  const absl::string_view test_name = "Comparison";
+
+  If<PropertiesOfT<Prof>::is_equality_comparable>::Invoke(
+      ExpectNotEq{errors}, test_name, context, small, big, small_name,
+      big_name);
+  If<PropertiesOfT<Prof>::is_equality_comparable>::Invoke(
+      ExpectNotEq{errors}, test_name, context, big, small, big_name,
+      small_name);
+
+  If<PropertiesOfT<Prof>::is_inequality_comparable>::Invoke(
+      ExpectNe{errors}, test_name, context, small, big, small_name, big_name);
+  If<PropertiesOfT<Prof>::is_inequality_comparable>::Invoke(
+      ExpectNe{errors}, test_name, context, big, small, big_name, small_name);
+
+  If<PropertiesOfT<Prof>::is_less_than_comparable>::Invoke(
+      ExpectLt{errors}, test_name, context, small, big, small_name, big_name);
+  If<PropertiesOfT<Prof>::is_less_than_comparable>::Invoke(
+      ExpectNotLt{errors}, test_name, context, big, small, big_name,
+      small_name);
+
+  If<PropertiesOfT<Prof>::is_less_equal_comparable>::Invoke(
+      ExpectLe{errors}, test_name, context, small, big, small_name, big_name);
+  If<PropertiesOfT<Prof>::is_less_equal_comparable>::Invoke(
+      ExpectNotLe{errors}, test_name, context, big, small, big_name,
+      small_name);
+
+  If<PropertiesOfT<Prof>::is_greater_equal_comparable>::Invoke(
+      ExpectNotGe{errors}, test_name, context, small, big, small_name,
+      big_name);
+  If<PropertiesOfT<Prof>::is_greater_equal_comparable>::Invoke(
+      ExpectGe{errors}, test_name, context, big, small, big_name, small_name);
+
+  If<PropertiesOfT<Prof>::is_greater_than_comparable>::Invoke(
+      ExpectNotGt{errors}, test_name, context, small, big, small_name,
+      big_name);
+  If<PropertiesOfT<Prof>::is_greater_than_comparable>::Invoke(
+      ExpectGt{errors}, test_name, context, big, small, big_name, small_name);
+}
+
+// For every two elements of an equivalence class, makes sure that those two
+// elements compare equal, including checks with the same argument passed as
+// both operands.
+template <class T, class Prof>
+struct ExpectEquivalenceClassComparisons {
+  template <class... Funs>
+  void operator()(EquivalenceClassType<Funs...> eq_class) const {
+    (ForEachTupleElement)(ExpectSelfComparison<T, Prof>{errors},
+                          eq_class.generators);
+
+    (ForEveryTwo)(ExpectEquivalenceClassComparison<T, Prof>{errors},
+                  eq_class.generators);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// For every element of an equivalence class, makes sure that the element is
+// self-consistent (in other words, if any of move/copy/swap are defined,
+// perform those operations and make such that results and operands still
+// compare equal to known values whenever it is required for that operation.
+template <class T, class Prof>
+struct ExpectEquivalenceClass {
+  template <class... Funs>
+  void operator()(EquivalenceClassType<Funs...> eq_class) const {
+    (ForEachTupleElement)(ExpectConsistency<T, Prof>{errors},
+                          eq_class.generators);
+
+    (ForEveryTwo)(ExpectEquivalenceClassConsistency<T, Prof>{errors},
+                  eq_class.generators);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Validate that the passed-in argument is a generator of a greater value than
+// the one produced by the "small_gen" datamember with respect to all of the
+// comparison operators that Prof requires, with both argument orders to test.
+template <class T, class Prof, class SmallGenerator>
+struct ExpectBiggerGeneratorThanComparisons {
+  template <class BigGenerator>
+  void operator()(BigGenerator big_gen) const {
+    const T small = small_gen();
+    const T big = big_gen();
+
+    (ExpectOrdered<T, Prof>)(errors,
+                             PrepareGivenContext(
+                                 GivenDeclaration{"const _T small",
+                                                  small_gen.description},
+                                 GivenDeclaration{"const _T big",
+                                                  big_gen.description}),
+                             small, big, "small", "big");
+  }
+
+  SmallGenerator small_gen;
+  ConformanceErrors* errors;
+};
+
+// Perform all of the move, copy, and swap checks on the value generated by
+// `small_gen` and the value generated by `big_gen`.
+template <class T, class Prof, class SmallGenerator>
+struct ExpectBiggerGeneratorThan {
+  template <class BigGenerator>
+  void operator()(BigGenerator big_gen) const {
+    If<PropertiesOfT<Prof>::is_move_assignable>::Invoke(
+        ExpectMoveAssign<T, Prof>{errors}, small_gen, big_gen);
+    If<PropertiesOfT<Prof>::is_move_assignable>::Invoke(
+        ExpectMoveAssign<T, Prof>{errors}, big_gen, small_gen);
+
+    If<PropertiesOfT<Prof>::is_copy_assignable>::Invoke(
+        ExpectCopyAssign<T, Prof>{errors}, small_gen, big_gen);
+    If<PropertiesOfT<Prof>::is_copy_assignable>::Invoke(
+        ExpectCopyAssign<T, Prof>{errors}, big_gen, small_gen);
+
+    If<PropertiesOfT<Prof>::is_swappable>::Invoke(ExpectSwap<T, Prof>{errors},
+                                                  small_gen, big_gen);
+  }
+
+  SmallGenerator small_gen;
+  ConformanceErrors* errors;
+};
+
+// Validate that the result of a generator is greater than the results of all
+// generators in an equivalence class with respect to comparisons.
+template <class T, class Prof, class SmallGenerator>
+struct ExpectBiggerGeneratorThanEqClassesComparisons {
+  template <class BigEqClass>
+  void operator()(BigEqClass big_eq_class) const {
+    (ForEachTupleElement)(
+        ExpectBiggerGeneratorThanComparisons<T, Prof, SmallGenerator>{small_gen,
+                                                                      errors},
+        big_eq_class.generators);
+  }
+
+  SmallGenerator small_gen;
+  ConformanceErrors* errors;
+};
+
+// Validate that the non-comparison binary operations required by Prof are
+// correct for the result of each generator of big_eq_class and a generator of
+// the logically smaller value returned by small_gen.
+template <class T, class Prof, class SmallGenerator>
+struct ExpectBiggerGeneratorThanEqClasses {
+  template <class BigEqClass>
+  void operator()(BigEqClass big_eq_class) const {
+    (ForEachTupleElement)(
+        ExpectBiggerGeneratorThan<T, Prof, SmallGenerator>{small_gen, errors},
+        big_eq_class.generators);
+  }
+
+  SmallGenerator small_gen;
+  ConformanceErrors* errors;
+};
+
+// Validate that each equivalence class that is passed is logically less than
+// the equivalence classes that comes later on in the argument list.
+template <class T, class Prof>
+struct ExpectOrderedEquivalenceClassesComparisons {
+  template <class... BigEqClasses>
+  struct Impl {
+    // Validate that the value produced by `small_gen` is less than all of the
+    // values generated by those of the logically larger equivalence classes.
+    template <class SmallGenerator>
+    void operator()(SmallGenerator small_gen) const {
+      (ForEachTupleElement)(ExpectBiggerGeneratorThanEqClassesComparisons<
+                                T, Prof, SmallGenerator>{small_gen, errors},
+                            big_eq_classes);
+    }
+
+    std::tuple<BigEqClasses...> big_eq_classes;
+    ConformanceErrors* errors;
+  };
+
+  // When given no equivalence classes, no validation is necessary.
+  void operator()() const {}
+
+  template <class SmallEqClass, class... BigEqClasses>
+  void operator()(SmallEqClass small_eq_class,
+                  BigEqClasses... big_eq_classes) const {
+    // For each generator in the first equivalence class, make sure that it is
+    // less than each of those in the logically greater equivalence classes.
+    (ForEachTupleElement)(
+        Impl<BigEqClasses...>{std::make_tuple(absl::move(big_eq_classes)...),
+                              errors},
+        small_eq_class.generators);
+
+    // Recurse so that all equivalence class combinations are checked.
+    (*this)(absl::move(big_eq_classes)...);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Validate that the non-comparison binary operations required by Prof are
+// correct for the result of each generator of big_eq_classes and a generator of
+// the logically smaller value returned by small_gen.
+template <class T, class Prof>
+struct ExpectOrderedEquivalenceClasses {
+  template <class... BigEqClasses>
+  struct Impl {
+    template <class SmallGenerator>
+    void operator()(SmallGenerator small_gen) const {
+      (ForEachTupleElement)(
+          ExpectBiggerGeneratorThanEqClasses<T, Prof, SmallGenerator>{small_gen,
+                                                                      errors},
+          big_eq_classes);
+    }
+
+    std::tuple<BigEqClasses...> big_eq_classes;
+    ConformanceErrors* errors;
+  };
+
+  // Check that small_eq_class is logically consistent and also is logically
+  // less than all values in big_eq_classes.
+  template <class SmallEqClass, class... BigEqClasses>
+  void operator()(SmallEqClass small_eq_class,
+                  BigEqClasses... big_eq_classes) const {
+    (ForEachTupleElement)(
+        Impl<BigEqClasses...>{std::make_tuple(absl::move(big_eq_classes)...),
+                              errors},
+        small_eq_class.generators);
+
+    (*this)(absl::move(big_eq_classes)...);
+  }
+
+  // Terminating case of operator().
+  void operator()() const {}
+
+  ConformanceErrors* errors;
+};
+
+// Validate that a type meets the syntactic requirements of std::hash if the
+// range of profiles requires it.
+template <class T, class MinProf, class MaxProf>
+struct ExpectHashable {
+  void operator()() const {
+    ExpectModelOfHashable<T, MinProf, MaxProf>(errors);
+  }
+
+  ConformanceErrors* errors;
+};
+
+// Validate that the type `T` meets all of the requirements associated with
+// `MinProf` and without going beyond the syntactic properties of `MaxProf`.
+template <class T, class MinProf, class MaxProf>
+struct ExpectModels {
+  void operator()(ConformanceErrors* errors) const {
+    ExpectModelOfDefaultConstructible<T, MinProf, MaxProf>(errors);
+    ExpectModelOfMoveConstructible<T, MinProf, MaxProf>(errors);
+    ExpectModelOfCopyConstructible<T, MinProf, MaxProf>(errors);
+    ExpectModelOfMoveAssignable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfCopyAssignable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfDestructible<T, MinProf, MaxProf>(errors);
+    ExpectModelOfEqualityComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfInequalityComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfLessThanComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfLessEqualComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfGreaterEqualComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfGreaterThanComparable<T, MinProf, MaxProf>(errors);
+    ExpectModelOfSwappable<T, MinProf, MaxProf>(errors);
+
+    // Only check hashability on compilers that have a compliant default-hash.
+    If<!poisoned_hash_fails_instantiation()>::Invoke(
+        ExpectHashable<T, MinProf, MaxProf>{errors});
+  }
+};
+
+// A metafunction that yields a Profile matching the set of properties that are
+// safe to be checked (lack-of-hashability is only checked on standard library
+// implementations that are standards compliant in that they provide a std::hash
+// primary template that is SFINAE-friendly)
+template <class LogicalProf, class T>
+struct MinimalCheckableProfile {
+  using type =
+      MinimalProfiles<PropertiesOfT<LogicalProf>,
+                      PropertiesOfT<SyntacticConformanceProfileOf<
+                          T, !PropertiesOfT<LogicalProf>::is_hashable &&
+                                     poisoned_hash_fails_instantiation()
+                                 ? CheckHashability::no
+                                 : CheckHashability::yes>>>;
+};
+
+// An identity metafunction
+template <class T>
+struct Always {
+  using type = T;
+};
+
+// Validate the T meets all of the necessary requirements of LogicalProf, with
+// syntactic requirements defined by the profile range [MinProf, MaxProf].
+template <class T, class LogicalProf, class MinProf, class MaxProf,
+          class... EqClasses>
+ConformanceErrors ExpectRegularityImpl(
+    OrderedEquivalenceClasses<EqClasses...> vals) {
+  ConformanceErrors errors((NameOf<T>()));
+
+  If<!constexpr_instantiation_when_unevaluated()>::Invoke(
+      ExpectModels<T, MinProf, MaxProf>(), &errors);
+
+  using minimal_profile = typename absl::conditional_t<
+      constexpr_instantiation_when_unevaluated(), Always<LogicalProf>,
+      MinimalCheckableProfile<LogicalProf, T>>::type;
+
+  If<PropertiesOfT<minimal_profile>::is_default_constructible>::Invoke(
+      ExpectDefaultConstructWithDestruct<T>{&errors});
+
+  //////////////////////////////////////////////////////////////////////////////
+  // Perform all comparison checks first, since later checks depend on their
+  // correctness.
+  //
+  // Check all of the comparisons for all values in the same equivalence
+  // class (equal with respect to comparison operators and hash the same).
+  (ForEachTupleElement)(
+      ExpectEquivalenceClassComparisons<T, minimal_profile>{&errors},
+      vals.eq_classes);
+
+  // Check all of the comparisons for each combination of values that are in
+  // different equivalence classes (not equal with respect to comparison
+  // operators).
+  absl::apply(
+      ExpectOrderedEquivalenceClassesComparisons<T, minimal_profile>{&errors},
+      vals.eq_classes);
+  //
+  //////////////////////////////////////////////////////////////////////////////
+
+  // Perform remaining checks, relying on comparisons.
+  // TODO(calabrese) short circuit if any comparisons above failed.
+  (ForEachTupleElement)(ExpectEquivalenceClass<T, minimal_profile>{&errors},
+                        vals.eq_classes);
+
+  absl::apply(ExpectOrderedEquivalenceClasses<T, minimal_profile>{&errors},
+              vals.eq_classes);
+
+  return errors;
+}
+
+// A type that represents a range of profiles that are acceptable to be matched.
+//
+// `MinProf` is the minimum set of syntactic requirements that must be met.
+//
+// `MaxProf` is the maximum set of syntactic requirements that must be met.
+// This maximum is particularly useful for certain "strictness" checking. Some
+// examples for when this is useful:
+//
+// * Making sure that a type is move-only (rather than simply movable)
+//
+// * Making sure that a member function is *not* noexcept in cases where it
+//   cannot be noexcept, such as if a dependent datamember has certain
+//   operations that are not noexcept.
+//
+// * Making sure that a type tightly matches a spec, such as the standard.
+//
+// `LogicalProf` is the Profile for which run-time testing is to take place.
+//
+// Note: The reason for `LogicalProf` is because it is often the case, when
+// dealing with templates, that a declaration of a given operation is specified,
+// but whose body would fail to instantiate. Examples include the
+// copy-constructor of a standard container when the element-type is move-only,
+// or the comparison operators of a standard container when the element-type
+// does not have the necessary comparison operations defined. The `LogicalProf`
+// parameter allows us to capture the intent of what should be tested at
+// run-time, even in the cases where syntactically it might otherwise appear as
+// though the type undergoing testing supports more than it actually does.
+template <class LogicalProf, class MinProf = LogicalProf,
+          class MaxProf = MinProf>
+struct ProfileRange {
+  using logical_profile = LogicalProf;
+  using min_profile = MinProf;
+  using max_profile = MaxProf;
+};
+
+// Similar to ProfileRange except that it creates a profile range that is
+// coupled with a Domain and is used when testing that a type matches exactly
+// the "minimum" requirements of LogicalProf.
+template <class StrictnessDomain, class LogicalProf,
+          class MinProf = LogicalProf, class MaxProf = MinProf>
+struct StrictProfileRange {
+  // We do not yet support extension.
+  static_assert(
+      std::is_same<StrictnessDomain, RegularityDomain>::value,
+      "Currently, the only valid StrictnessDomain is RegularityDomain.");
+  using strictness_domain = StrictnessDomain;
+  using logical_profile = LogicalProf;
+  using min_profile = MinProf;
+  using max_profile = MaxProf;
+};
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction that creates a StrictProfileRange from a Domain and either a
+// Profile or ProfileRange.
+template <class StrictnessDomain, class ProfOrRange>
+struct MakeStrictProfileRange;
+
+template <class StrictnessDomain, class LogicalProf>
+struct MakeStrictProfileRange {
+  using type = StrictProfileRange<StrictnessDomain, LogicalProf>;
+};
+
+template <class StrictnessDomain, class LogicalProf, class MinProf,
+          class MaxProf>
+struct MakeStrictProfileRange<StrictnessDomain,
+                              ProfileRange<LogicalProf, MinProf, MaxProf>> {
+  using type =
+      StrictProfileRange<StrictnessDomain, LogicalProf, MinProf, MaxProf>;
+};
+
+template <class StrictnessDomain, class ProfOrRange>
+using MakeStrictProfileRangeT =
+    typename MakeStrictProfileRange<StrictnessDomain, ProfOrRange>::type;
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A profile in the RegularityDomain with the strongest possible requirements.
+using MostStrictProfile =
+    CombineProfiles<TriviallyCompleteProfile, NothrowComparableProfile>;
+
+// Forms a ProfileRange that treats the Profile as the bare minimum requirements
+// of a type.
+template <class LogicalProf, class MinProf = LogicalProf>
+using LooseProfileRange = StrictProfileRange<RegularityDomain, LogicalProf,
+                                             MinProf, MostStrictProfile>;
+
+template <class Prof>
+using MakeLooseProfileRangeT = Prof;
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// The following classes implement the metafunction ProfileRangeOfT<T> that
+// takes either a Profile or ProfileRange and yields the ProfileRange to be
+// used during testing.
+//
+template <class T, class /*Enabler*/ = void>
+struct ProfileRangeOfImpl;
+
+template <class T>
+struct ProfileRangeOfImpl<T, absl::void_t<PropertiesOfT<T>>> {
+  using type = LooseProfileRange<T>;
+};
+
+template <class T>
+struct ProfileRangeOf : ProfileRangeOfImpl<T> {};
+
+template <class StrictnessDomain, class LogicalProf, class MinProf,
+          class MaxProf>
+struct ProfileRangeOf<
+    StrictProfileRange<StrictnessDomain, LogicalProf, MinProf, MaxProf>> {
+  using type =
+      StrictProfileRange<StrictnessDomain, LogicalProf, MinProf, MaxProf>;
+};
+
+template <class T>
+using ProfileRangeOfT = typename ProfileRangeOf<T>::type;
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// Extract the logical profile of a range (what will be runtime tested).
+template <class T>
+using LogicalProfileOfT = typename ProfileRangeOfT<T>::logical_profile;
+
+// Extract the minimal syntactic profile of a range (error if not at least).
+template <class T>
+using MinProfileOfT = typename ProfileRangeOfT<T>::min_profile;
+
+// Extract the maximum syntactic profile of a range (error if more than).
+template <class T>
+using MaxProfileOfT = typename ProfileRangeOfT<T>::max_profile;
+
+////////////////////////////////////////////////////////////////////////////////
+//
+template <class T>
+struct IsProfileOrProfileRange : IsProfile<T>::type {};
+
+template <class StrictnessDomain, class LogicalProf, class MinProf,
+          class MaxProf>
+struct IsProfileOrProfileRange<
+    StrictProfileRange<StrictnessDomain, LogicalProf, MinProf, MaxProf>>
+    : std::true_type {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// TODO(calabrese): Consider naming the functions in this class the same as
+// the macros (defined later on) so that auto-complete leads to the correct name
+// and so that a user cannot accidentally call a function rather than the macro
+// form.
+template <bool ExpectSuccess, class T, class... EqClasses>
+struct ExpectConformanceOf {
+  // Add a value to be tested. Subsequent calls to this function on the same
+  // object must specify logically "larger" values with respect to the
+  // comparison operators of the type, if any.
+  //
+  // NOTE: This function should not be called directly. A stateless lambda is
+  // implicitly formed and passed when using the INITIALIZER macro at the bottom
+  // of this file.
+  template <class Fun,
+            absl::enable_if_t<std::is_same<
+                ResultOfGeneratorT<GeneratorType<Fun>>, T>::value>** = nullptr>
+  ABSL_MUST_USE_RESULT ExpectConformanceOf<ExpectSuccess, T, EqClasses...,
+                                           EquivalenceClassType<Fun>>
+  initializer(GeneratorType<Fun> fun) && {
+    return {
+        {std::tuple_cat(absl::move(ordered_vals.eq_classes),
+                        std::make_tuple((EquivalenceClass)(absl::move(fun))))},
+        std::move(expected_failed_tests)};
+  }
+
+  template <class... TestNames,
+            absl::enable_if_t<!ExpectSuccess && sizeof...(EqClasses) == 0 &&
+                              absl::conjunction<std::is_convertible<
+                                  TestNames, absl::string_view>...>::value>** =
+                nullptr>
+  ABSL_MUST_USE_RESULT ExpectConformanceOf<ExpectSuccess, T, EqClasses...>
+  due_to(TestNames&&... test_names) && {
+    (InsertEach)(&expected_failed_tests,
+                 absl::AsciiStrToLower(absl::string_view(test_names))...);
+
+    return {absl::move(ordered_vals), std::move(expected_failed_tests)};
+  }
+
+  template <class... TestNames, int = 0,  // MSVC disambiguator
+            absl::enable_if_t<ExpectSuccess && sizeof...(EqClasses) == 0 &&
+                              absl::conjunction<std::is_convertible<
+                                  TestNames, absl::string_view>...>::value>** =
+                nullptr>
+  ABSL_MUST_USE_RESULT ExpectConformanceOf<ExpectSuccess, T, EqClasses...>
+  due_to(TestNames&&... test_names) && {
+    // TODO(calabrese) Instead have DUE_TO only exist via a CRTP base.
+    // This would produce better errors messages than the static_assert.
+    static_assert(!ExpectSuccess,
+                  "DUE_TO cannot be called when conformance is expected -- did "
+                  "you mean to use ASSERT_NONCONFORMANCE_OF?");
+  }
+
+  // Add a value to be tested. Subsequent calls to this function on the same
+  // object must specify logically "larger" values with respect to the
+  // comparison operators of the type, if any.
+  //
+  // NOTE: This function should not be called directly. A stateful lambda is
+  // implicitly formed and passed when using the INITIALIZER macro at the bottom
+  // of this file.
+  template <class Fun,
+            absl::enable_if_t<std::is_same<
+                ResultOfGeneratorT<GeneratorType<Fun>>, T>::value>** = nullptr>
+  ABSL_MUST_USE_RESULT ExpectConformanceOf<ExpectSuccess, T, EqClasses...,
+                                           EquivalenceClassType<Fun>>
+  dont_class_directly_stateful_initializer(GeneratorType<Fun> fun) && {
+    return {
+        {std::tuple_cat(absl::move(ordered_vals.eq_classes),
+                        std::make_tuple((EquivalenceClass)(absl::move(fun))))},
+        std::move(expected_failed_tests)};
+  }
+
+  // Add a set of value to be tested, where each value is equal with respect to
+  // the comparison operators and std::hash specialization, if defined.
+  template <
+      class... Funs,
+      absl::void_t<absl::enable_if_t<std::is_same<
+          ResultOfGeneratorT<GeneratorType<Funs>>, T>::value>...>** = nullptr>
+  ABSL_MUST_USE_RESULT ExpectConformanceOf<ExpectSuccess, T, EqClasses...,
+                                           EquivalenceClassType<Funs...>>
+  equivalence_class(GeneratorType<Funs>... funs) && {
+    return {{std::tuple_cat(
+                absl::move(ordered_vals.eq_classes),
+                std::make_tuple((EquivalenceClass)(absl::move(funs)...)))},
+            std::move(expected_failed_tests)};
+  }
+
+  // Execute the tests for the captured set of values, strictly matching a range
+  // of expected profiles in a given domain.
+  template <
+      class ProfRange,
+      absl::enable_if_t<IsProfileOrProfileRange<ProfRange>::value>** = nullptr>
+  ABSL_MUST_USE_RESULT ::testing::AssertionResult with_strict_profile(
+      ProfRange /*profile*/) {
+    ConformanceErrors test_result =
+        (ExpectRegularityImpl<
+            T, LogicalProfileOfT<ProfRange>, MinProfileOfT<ProfRange>,
+            MaxProfileOfT<ProfRange>>)(absl::move(ordered_vals));
+
+    return ExpectSuccess ? test_result.assertionResult()
+                         : test_result.expectFailedTests(expected_failed_tests);
+  }
+
+  // Execute the tests for the captured set of values, loosely matching a range
+  // of expected profiles (loose in that an interface is allowed to be more
+  // refined that a profile suggests, such as a type having a noexcept copy
+  // constructor when all that is required is that the copy constructor exists).
+  template <class Prof, absl::enable_if_t<IsProfile<Prof>::value>** = nullptr>
+  ABSL_MUST_USE_RESULT ::testing::AssertionResult with_loose_profile(
+      Prof /*profile*/) {
+    ConformanceErrors test_result =
+        (ExpectRegularityImpl<
+            T, Prof, Prof,
+            CombineProfiles<TriviallyCompleteProfile,
+                            NothrowComparableProfile>>)(absl::
+                                                            move(ordered_vals));
+
+    return ExpectSuccess ? test_result.assertionResult()
+                         : test_result.expectFailedTests(expected_failed_tests);
+  }
+
+  OrderedEquivalenceClasses<EqClasses...> ordered_vals;
+  std::set<std::string> expected_failed_tests;
+};
+
+template <class T>
+using ExpectConformanceOfType = ExpectConformanceOf</*ExpectSuccess=*/true, T>;
+
+template <class T>
+using ExpectNonconformanceOfType =
+    ExpectConformanceOf</*ExpectSuccess=*/false, T>;
+
+struct EquivalenceClassMaker {
+  // TODO(calabrese) Constrain to callable
+  template <class Fun>
+  static GeneratorType<Fun> initializer(GeneratorType<Fun> fun) {
+    return fun;
+  }
+};
+
+// A top-level macro that begins the builder pattern.
+//
+// The argument here takes the datatype to be tested.
+#define ABSL_INTERNAL_ASSERT_CONFORMANCE_OF(...)                            \
+  GTEST_AMBIGUOUS_ELSE_BLOCKER_                                             \
+  if ABSL_INTERNAL_LPAREN                                                   \
+  const ::testing::AssertionResult gtest_ar =                               \
+      ABSL_INTERNAL_LPAREN ::absl::types_internal::ExpectConformanceOfType< \
+          __VA_ARGS__>()
+
+// Akin to ASSERT_CONFORMANCE_OF except that it expects failure and tries to
+// match text.
+#define ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(...)                            \
+  GTEST_AMBIGUOUS_ELSE_BLOCKER_                                                \
+  if ABSL_INTERNAL_LPAREN                                                      \
+  const ::testing::AssertionResult gtest_ar =                                  \
+      ABSL_INTERNAL_LPAREN ::absl::types_internal::ExpectNonconformanceOfType< \
+          __VA_ARGS__>()
+
+////////////////////////////////////////////////////////////////////////////////
+// NOTE: The following macros look like they are recursive, but are not (macros
+// cannot recurse). These actually refer to member functions of the same name.
+// This is done intentionally so that a user cannot accidentally invoke a
+// member function of the conformance-testing suite without going through the
+// macro.
+////////////////////////////////////////////////////////////////////////////////
+
+// Specify expected test failures as comma-separated strings.
+#define DUE_TO(...) due_to(__VA_ARGS__)
+
+// Specify a value to be tested.
+//
+// Note: Internally, this takes an expression and turns it into the return value
+// of lambda that captures no data. The expression is stringized during
+// preprocessing so that it can be used in error reports.
+#define INITIALIZER(...)                         \
+  initializer(::absl::types_internal::Generator( \
+      [] { return __VA_ARGS__; }, ABSL_INTERNAL_STRINGIZE(__VA_ARGS__)))
+
+// Specify a value to be tested.
+//
+// Note: Internally, this takes an expression and turns it into the return value
+// of lambda that captures data by reference. The expression is stringized
+// during preprocessing so that it can be used in error reports.
+#define STATEFUL_INITIALIZER(...)                         \
+  stateful_initializer(::absl::types_internal::Generator( \
+      [&] { return __VA_ARGS__; }, ABSL_INTERNAL_STRINGIZE(__VA_ARGS__)))
+
+// Used in the builder-pattern.
+//
+// Takes a series of INITIALIZER and/or STATEFUL_INITIALIZER invocations and
+// forwards them along to be tested, grouping them such that the testing suite
+// knows that they are supposed to represent the same logical value (the values
+// compare the same, hash the same, etc.).
+#define EQUIVALENCE_CLASS(...)                    \
+  equivalence_class(ABSL_INTERNAL_TRANSFORM_ARGS( \
+      ABSL_INTERNAL_PREPEND_EQ_MAKER, __VA_ARGS__))
+
+// An invocation of this or WITH_STRICT_PROFILE must end the builder-pattern.
+// It takes a Profile as its argument.
+//
+// This executes the tests and allows types that are "more referined" than the
+// profile specifies, but not less. For instance, if the Profile specifies
+// noexcept copy-constructiblity, the test will fail if the copy-constructor is
+// not noexcept, however, it will succeed if the copy constructor is trivial.
+//
+// This is useful for testing that a type meets some minimum set of
+// requirements.
+#define WITH_LOOSE_PROFILE(...)                                      \
+  with_loose_profile(                                                \
+      ::absl::types_internal::MakeLooseProfileRangeT<__VA_ARGS__>()) \
+      ABSL_INTERNAL_RPAREN ABSL_INTERNAL_RPAREN;                     \
+  else GTEST_FATAL_FAILURE_(gtest_ar.failure_message())  // NOLINT
+
+// An invocation of this or WITH_STRICT_PROFILE must end the builder-pattern.
+// It takes a Domain and a Profile as its arguments.
+//
+// This executes the tests and disallows types that differ at all from the
+// properties of the Profile. For instance, if the Profile specifies noexcept
+// copy-constructiblity, the test will fail if the copy constructor is trivial.
+//
+// This is useful for testing that a type does not do anything more than a
+// specification requires, such as to minimize things like Hyrum's Law, or more
+// commonly, to prevent a type from being "accidentally" copy-constructible in
+// a way that may produce incorrect results, simply because the user forget to
+// delete that operation.
+#define WITH_STRICT_PROFILE(...)                                      \
+  with_strict_profile(                                                \
+      ::absl::types_internal::MakeStrictProfileRangeT<__VA_ARGS__>()) \
+      ABSL_INTERNAL_RPAREN ABSL_INTERNAL_RPAREN;                      \
+  else GTEST_FATAL_FAILURE_(gtest_ar.failure_message())  // NOLINT
+
+// Internal macro that is used in the internals of the EDSL when forming
+// equivalence classes.
+#define ABSL_INTERNAL_PREPEND_EQ_MAKER(arg) \
+  ::absl::types_internal::EquivalenceClassMaker().arg
+
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_H_

diff --git a/absl/types/internal/conformance_testing_helpers.h b/absl/types/internal/conformance_testing_helpers.h
new file mode 100644
index 0000000..00775f9
--- /dev/null
+++ b/absl/types/internal/conformance_testing_helpers.h

@@ -0,0 +1,391 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_HELPERS_H_
+#define ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_HELPERS_H_
+
+// Checks to determine whether or not we can use abi::__cxa_demangle
+#if (defined(__ANDROID__) || defined(ANDROID)) && !defined(OS_ANDROID)
+#define ABSL_INTERNAL_OS_ANDROID
+#endif
+
+// We support certain compilers only.  See demangle.h for details.
+#if defined(OS_ANDROID) && (defined(__i386__) || defined(__x86_64__))
+#define ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE 0
+#elif (__GNUC__ >= 4 || (__GNUC__ >= 3 && __GNUC_MINOR__ >= 4)) && \
+    !defined(__mips__)
+#define ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE 1
+#elif defined(__clang__) && !defined(_MSC_VER)
+#define ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE 1
+#else
+#define ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE 0
+#endif
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+#include "absl/utility/utility.h"
+
+#if ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE
+#include <cxxabi.h>
+
+#include <cstdlib>
+#endif
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+
+// Return a readable name for type T.
+template <class T>
+absl::string_view NameOfImpl() {
+// TODO(calabrese) Investigate using debugging:internal_demangle as a fallback.
+#if ABSL_TYPES_INTERNAL_HAS_CXA_DEMANGLE
+  int status = 0;
+  char* demangled_name = nullptr;
+
+  demangled_name =
+      abi::__cxa_demangle(typeid(T).name(), nullptr, nullptr, &status);
+
+  if (status == 0 && demangled_name != nullptr) {
+    return demangled_name;
+  } else {
+    return typeid(T).name();
+  }
+#else
+  return typeid(T).name();
+#endif
+  // NOTE: We intentionally leak demangled_name so that it remains valid
+  // throughout the remainder of the program.
+}
+
+// Given a type, returns as nice of a type name as we can produce (demangled).
+//
+// Note: This currently strips cv-qualifiers and references, but that is okay
+// because we only use this internally with unqualified object types.
+template <class T>
+std::string NameOf() {
+  static const absl::string_view result = NameOfImpl<T>();
+  return std::string(result);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Metafunction to check if a type is callable with no explicit arguments
+template <class Fun, class /*Enabler*/ = void>
+struct IsNullaryCallableImpl : std::false_type {};
+
+template <class Fun>
+struct IsNullaryCallableImpl<
+    Fun, absl::void_t<decltype(std::declval<const Fun&>()())>>
+    : std::true_type {
+  using result_type = decltype(std::declval<const Fun&>()());
+
+  template <class ValueType>
+  using for_type = std::is_same<ValueType, result_type>;
+
+  using void_if_true = void;
+};
+
+template <class Fun>
+struct IsNullaryCallable : IsNullaryCallableImpl<Fun> {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A type that contains a function object that returns an instance of a type
+// that is undergoing conformance testing. This function is required to always
+// return the same value upon invocation.
+template <class Fun>
+struct GeneratorType;
+
+// A type that contains a tuple of GeneratorType<Fun> where each Fun has the
+// same return type. The result of each of the different generators should all
+// be equal values, though the underlying object representation may differ (such
+// as if one returns 0.0 and another return -0.0, or if one returns an empty
+// vector and another returns an empty vector with a different capacity.
+template <class... Funs>
+struct EquivalenceClassType;
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction to check if a type is a specialization of EquivalenceClassType
+template <class T>
+struct IsEquivalenceClass : std::false_type {};
+
+template <>
+struct IsEquivalenceClass<EquivalenceClassType<>> : std::true_type {
+  using self = IsEquivalenceClass;
+
+  // A metafunction to check if this EquivalenceClassType is a valid
+  // EquivalenceClassType for a type `ValueType` that is undergoing testing
+  template <class ValueType>
+  using for_type = std::true_type;
+};
+
+template <class Head, class... Tail>
+struct IsEquivalenceClass<EquivalenceClassType<Head, Tail...>>
+    : std::true_type {
+  using self = IsEquivalenceClass;
+
+  // The type undergoing conformance testing that this EquivalenceClass
+  // corresponds to
+  using result_type = typename IsNullaryCallable<Head>::result_type;
+
+  // A metafunction to check if this EquivalenceClassType is a valid
+  // EquivalenceClassType for a type `ValueType` that is undergoing testing
+  template <class ValueType>
+  using for_type = std::is_same<ValueType, result_type>;
+};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// A type that contains an ordered series of EquivalenceClassTypes, where the
+// the function object of each underlying GeneratorType has the same return type
+//
+// These equivalence classes are required to be in a logical ascending order
+// that is consistent with comparison operators that are defined for the return
+// type of each GeneratorType, if any.
+template <class... EqClasses>
+struct OrderedEquivalenceClasses;
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction to determine the return type of the function object contained
+// in a GeneratorType specialization.
+template <class T>
+struct ResultOfGenerator {};
+
+template <class Fun>
+struct ResultOfGenerator<GeneratorType<Fun>> {
+  using type = decltype(std::declval<const Fun&>()());
+};
+
+template <class Fun>
+using ResultOfGeneratorT = typename ResultOfGenerator<GeneratorType<Fun>>::type;
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction that yields true iff each of Funs is a GeneratorType
+// specialization and they all contain functions with the same return type
+template <class /*Enabler*/, class... Funs>
+struct AreGeneratorsWithTheSameReturnTypeImpl : std::false_type {};
+
+template <>
+struct AreGeneratorsWithTheSameReturnTypeImpl<void> : std::true_type {};
+
+template <class Head, class... Tail>
+struct AreGeneratorsWithTheSameReturnTypeImpl<
+    typename std::enable_if<absl::conjunction<std::is_same<
+        ResultOfGeneratorT<Head>, ResultOfGeneratorT<Tail>>...>::value>::type,
+    Head, Tail...> : std::true_type {};
+
+template <class... Funs>
+struct AreGeneratorsWithTheSameReturnType
+    : AreGeneratorsWithTheSameReturnTypeImpl<void, Funs...>::type {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// A metafunction that yields true iff each of Funs is an EquivalenceClassType
+// specialization and they all contain GeneratorType specializations that have
+// the same return type
+template <class... EqClasses>
+struct AreEquivalenceClassesOfTheSameType {
+  static_assert(sizeof...(EqClasses) != sizeof...(EqClasses), "");
+};
+
+template <>
+struct AreEquivalenceClassesOfTheSameType<> : std::true_type {
+  using self = AreEquivalenceClassesOfTheSameType;
+
+  // Metafunction to check that a type is the same as all of the equivalence
+  // classes, if any.
+  // Note: In this specialization there are no equivalence classes, so the
+  // value type is always compatible.
+  template <class /*ValueType*/>
+  using for_type = std::true_type;
+};
+
+template <class... Funs>
+struct AreEquivalenceClassesOfTheSameType<EquivalenceClassType<Funs...>>
+    : std::true_type {
+  using self = AreEquivalenceClassesOfTheSameType;
+
+  // Metafunction to check that a type is the same as all of the equivalence
+  // classes, if any.
+  template <class ValueType>
+  using for_type = typename IsEquivalenceClass<
+      EquivalenceClassType<Funs...>>::template for_type<ValueType>;
+};
+
+template <class... TailEqClasses>
+struct AreEquivalenceClassesOfTheSameType<
+    EquivalenceClassType<>, EquivalenceClassType<>, TailEqClasses...>
+    : AreEquivalenceClassesOfTheSameType<TailEqClasses...>::self {};
+
+template <class HeadNextFun, class... TailNextFuns, class... TailEqClasses>
+struct AreEquivalenceClassesOfTheSameType<
+    EquivalenceClassType<>, EquivalenceClassType<HeadNextFun, TailNextFuns...>,
+    TailEqClasses...>
+    : AreEquivalenceClassesOfTheSameType<
+          EquivalenceClassType<HeadNextFun, TailNextFuns...>,
+          TailEqClasses...>::self {};
+
+template <class HeadHeadFun, class... TailHeadFuns, class... TailEqClasses>
+struct AreEquivalenceClassesOfTheSameType<
+    EquivalenceClassType<HeadHeadFun, TailHeadFuns...>, EquivalenceClassType<>,
+    TailEqClasses...>
+    : AreEquivalenceClassesOfTheSameType<
+          EquivalenceClassType<HeadHeadFun, TailHeadFuns...>,
+          TailEqClasses...>::self {};
+
+template <class HeadHeadFun, class... TailHeadFuns, class HeadNextFun,
+          class... TailNextFuns, class... TailEqClasses>
+struct AreEquivalenceClassesOfTheSameType<
+    EquivalenceClassType<HeadHeadFun, TailHeadFuns...>,
+    EquivalenceClassType<HeadNextFun, TailNextFuns...>, TailEqClasses...>
+    : absl::conditional_t<
+          IsNullaryCallable<HeadNextFun>::template for_type<
+              typename IsNullaryCallable<HeadHeadFun>::result_type>::value,
+          AreEquivalenceClassesOfTheSameType<
+              EquivalenceClassType<HeadHeadFun, TailHeadFuns...>,
+              TailEqClasses...>,
+          std::false_type> {};
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// Execute a function for each passed-in parameter.
+template <class Fun, class... Cases>
+void ForEachParameter(const Fun& fun, const Cases&... cases) {
+  const std::initializer_list<bool> results = {
+      (static_cast<void>(fun(cases)), true)...};
+
+  (void)results;
+}
+
+// Execute a function on each passed-in parameter (using a bound function).
+template <class Fun>
+struct ForEachParameterFun {
+  template <class... T>
+  void operator()(const T&... cases) const {
+    (ForEachParameter)(fun, cases...);
+  }
+
+  Fun fun;
+};
+
+// Execute a function on each element of a tuple.
+template <class Fun, class Tup>
+void ForEachTupleElement(const Fun& fun, const Tup& tup) {
+  absl::apply(ForEachParameterFun<Fun>{fun}, tup);
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Execute a function for each combination of two elements of a tuple, including
+// combinations of an element with itself.
+template <class Fun, class... T>
+struct ForEveryTwoImpl {
+  template <class Lhs>
+  struct WithBoundLhs {
+    template <class Rhs>
+    void operator()(const Rhs& rhs) const {
+      fun(lhs, rhs);
+    }
+
+    Fun fun;
+    Lhs lhs;
+  };
+
+  template <class Lhs>
+  void operator()(const Lhs& lhs) const {
+    (ForEachTupleElement)(WithBoundLhs<Lhs>{fun, lhs}, args);
+  }
+
+  Fun fun;
+  std::tuple<T...> args;
+};
+
+template <class Fun, class... T>
+void ForEveryTwo(const Fun& fun, std::tuple<T...> args) {
+  (ForEachTupleElement)(ForEveryTwoImpl<Fun, T...>{fun, args}, args);
+}
+//
+////////////////////////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Insert all values into an associative container
+template<class Container>
+void InsertEach(Container* cont) {
+}
+
+template<class Container, class H, class... T>
+void InsertEach(Container* cont, H&& head, T&&... tail) {
+  cont->insert(head);
+  (InsertEach)(cont, tail...);
+}
+//
+////////////////////////////////////////////////////////////////////////////////
+// A template with a nested "Invoke" static-member-function that executes a
+// passed-in Callable when `Condition` is true, otherwise it ignores the
+// Callable. This is useful for executing a function object with a condition
+// that corresponds to whether or not the Callable can be safely instantiated.
+// It has some overlapping uses with C++17 `if constexpr`.
+template <bool Condition>
+struct If;
+
+template <>
+struct If</*Condition =*/false> {
+  template <class Fun, class... P>
+  static void Invoke(const Fun& /*fun*/, P&&... /*args*/) {}
+};
+
+template <>
+struct If</*Condition =*/true> {
+  template <class Fun, class... P>
+  static void Invoke(const Fun& fun, P&&... args) {
+    // TODO(calabrese) Use std::invoke equivalent instead of function-call.
+    fun(absl::forward<P>(args)...);
+  }
+};
+
+//
+// ABSL_INTERNAL_STRINGIZE(...)
+//
+// This variadic macro transforms its arguments into a c-string literal after
+// expansion.
+//
+// Example:
+//
+//   ABSL_INTERNAL_STRINGIZE(std::array<int, 10>)
+//
+// Results in:
+//
+//   "std::array<int, 10>"
+#define ABSL_INTERNAL_STRINGIZE(...) ABSL_INTERNAL_STRINGIZE_IMPL((__VA_ARGS__))
+#define ABSL_INTERNAL_STRINGIZE_IMPL(arg) ABSL_INTERNAL_STRINGIZE_IMPL2 arg
+#define ABSL_INTERNAL_STRINGIZE_IMPL2(...) #__VA_ARGS__
+
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_TESTING_HELPERS_H_

diff --git a/absl/types/internal/conformance_testing_test.cc b/absl/types/internal/conformance_testing_test.cc
index 3dcf530..cf262fa 100644
--- a/absl/types/internal/conformance_testing_test.cc
+++ b/absl/types/internal/conformance_testing_test.cc

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "absl/types/internal/conformance_testing.h"
+
 #include <new>
 #include <type_traits>
 #include <utility>
@@ -19,6 +21,7 @@
 #include "gtest/gtest.h"
 #include "absl/meta/type_traits.h"
 #include "absl/types/internal/conformance_aliases.h"
+#include "absl/types/internal/conformance_profile.h"
 
 namespace {
 
@@ -1181,6 +1184,373 @@
                                CommonComparableProfilesToTest);
 INSTANTIATE_TYPED_TEST_SUITE_P(Trivial, ProfileTest, TrivialProfilesToTest);
 
-// TODO(calabrese) Test runtime results
+TEST(ConformanceTestingTest, Basic) {
+  using profile = ti::CombineProfiles<ti::TriviallyCompleteProfile,
+                                      ti::NothrowComparableProfile>;
+
+  using lim = std::numeric_limits<float>;
+
+  ABSL_INTERNAL_ASSERT_CONFORMANCE_OF(float)
+      .INITIALIZER(-lim::infinity())
+      .INITIALIZER(lim::lowest())
+      .INITIALIZER(-1.f)
+      .INITIALIZER(-lim::min())
+      .EQUIVALENCE_CLASS(INITIALIZER(-0.f), INITIALIZER(0.f))
+      .INITIALIZER(lim::min())
+      .INITIALIZER(1.f)
+      .INITIALIZER(lim::max())
+      .INITIALIZER(lim::infinity())
+      .WITH_STRICT_PROFILE(absl::types_internal::RegularityDomain, profile);
+}
+
+struct BadMoveConstruct {
+  BadMoveConstruct() = default;
+  BadMoveConstruct(BadMoveConstruct&& other) noexcept
+      : value(other.value + 1) {}
+  BadMoveConstruct& operator=(BadMoveConstruct&& other) noexcept = default;
+  int value = 0;
+
+  friend bool operator==(BadMoveConstruct const& lhs,
+                         BadMoveConstruct const& rhs) {
+    return lhs.value == rhs.value;
+  }
+  friend bool operator!=(BadMoveConstruct const& lhs,
+                         BadMoveConstruct const& rhs) {
+    return lhs.value != rhs.value;
+  }
+};
+
+struct BadMoveAssign {
+  BadMoveAssign() = default;
+  BadMoveAssign(BadMoveAssign&& other) noexcept = default;
+  BadMoveAssign& operator=(BadMoveAssign&& other) noexcept {
+    int new_value = other.value + 1;
+    value = new_value;
+    return *this;
+  }
+  int value = 0;
+
+  friend bool operator==(BadMoveAssign const& lhs, BadMoveAssign const& rhs) {
+    return lhs.value == rhs.value;
+  }
+  friend bool operator!=(BadMoveAssign const& lhs, BadMoveAssign const& rhs) {
+    return lhs.value != rhs.value;
+  }
+};
+
+enum class WhichCompIsBad { eq, ne, lt, le, ge, gt };
+
+template <WhichCompIsBad Which>
+struct BadCompare {
+  int value;
+
+  friend bool operator==(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::eq ? lhs.value != rhs.value
+                                       : lhs.value == rhs.value;
+  }
+
+  friend bool operator!=(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::ne ? lhs.value == rhs.value
+                                       : lhs.value != rhs.value;
+  }
+
+  friend bool operator<(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::lt ? lhs.value >= rhs.value
+                                       : lhs.value < rhs.value;
+  }
+
+  friend bool operator<=(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::le ? lhs.value > rhs.value
+                                       : lhs.value <= rhs.value;
+  }
+
+  friend bool operator>=(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::ge ? lhs.value < rhs.value
+                                       : lhs.value >= rhs.value;
+  }
+
+  friend bool operator>(BadCompare const& lhs, BadCompare const& rhs) {
+    return Which == WhichCompIsBad::gt ? lhs.value <= rhs.value
+                                       : lhs.value > rhs.value;
+  }
+};
+
+TEST(ConformanceTestingDeathTest, Failures) {
+  {
+    using profile = ti::CombineProfiles<ti::TriviallyCompleteProfile,
+                                        ti::NothrowComparableProfile>;
+
+    // Note: The initializers are intentionally in the wrong order.
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(float)
+        .INITIALIZER(1.f)
+        .INITIALIZER(0.f)
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using profile =
+        ti::CombineProfiles<ti::NothrowMovableProfile, ti::EquatableProfile>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadMoveConstruct)
+        .DUE_TO("Move construction")
+        .INITIALIZER(BadMoveConstruct())
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using profile =
+        ti::CombineProfiles<ti::NothrowMovableProfile, ti::EquatableProfile>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadMoveAssign)
+        .DUE_TO("Move assignment")
+        .INITIALIZER(BadMoveAssign())
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+TEST(ConformanceTestingDeathTest, CompFailures) {
+  using profile = ti::ComparableProfile;
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::eq>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::ne>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::lt>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::le>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::ge>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+
+  {
+    using BadComp = BadCompare<WhichCompIsBad::gt>;
+
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadComp)
+        .DUE_TO("Comparison")
+        .INITIALIZER(BadComp{0})
+        .INITIALIZER(BadComp{1})
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+struct BadSelfMove {
+  BadSelfMove() = default;
+  BadSelfMove(BadSelfMove&&) = default;
+  BadSelfMove& operator=(BadSelfMove&& other) noexcept {
+    if (this == &other) {
+      broken_state = true;
+    }
+    return *this;
+  }
+
+  friend bool operator==(const BadSelfMove& lhs, const BadSelfMove& rhs) {
+    return !(lhs.broken_state || rhs.broken_state);
+  }
+
+  friend bool operator!=(const BadSelfMove& lhs, const BadSelfMove& rhs) {
+    return lhs.broken_state || rhs.broken_state;
+  }
+
+  bool broken_state = false;
+};
+
+TEST(ConformanceTestingDeathTest, SelfMoveFailure) {
+  using profile = ti::EquatableNothrowMovableProfile;
+
+  {
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadSelfMove)
+        .DUE_TO("Move assignment")
+        .INITIALIZER(BadSelfMove())
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+struct BadSelfCopy {
+  BadSelfCopy() = default;
+  BadSelfCopy(BadSelfCopy&&) = default;
+  BadSelfCopy(const BadSelfCopy&) = default;
+  BadSelfCopy& operator=(BadSelfCopy&&) = default;
+  BadSelfCopy& operator=(BadSelfCopy const& other) {
+    if (this == &other) {
+      broken_state = true;
+    }
+    return *this;
+  }
+
+  friend bool operator==(const BadSelfCopy& lhs, const BadSelfCopy& rhs) {
+    return !(lhs.broken_state || rhs.broken_state);
+  }
+
+  friend bool operator!=(const BadSelfCopy& lhs, const BadSelfCopy& rhs) {
+    return lhs.broken_state || rhs.broken_state;
+  }
+
+  bool broken_state = false;
+};
+
+TEST(ConformanceTestingDeathTest, SelfCopyFailure) {
+  using profile = ti::EquatableValueProfile;
+
+  {
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadSelfCopy)
+        .DUE_TO("Copy assignment")
+        .INITIALIZER(BadSelfCopy())
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+struct BadSelfSwap {
+  friend void swap(BadSelfSwap& lhs, BadSelfSwap& rhs) noexcept {
+    if (&lhs == &rhs) lhs.broken_state = true;
+  }
+
+  friend bool operator==(const BadSelfSwap& lhs, const BadSelfSwap& rhs) {
+    return !(lhs.broken_state || rhs.broken_state);
+  }
+
+  friend bool operator!=(const BadSelfSwap& lhs, const BadSelfSwap& rhs) {
+    return lhs.broken_state || rhs.broken_state;
+  }
+
+  bool broken_state = false;
+};
+
+TEST(ConformanceTestingDeathTest, SelfSwapFailure) {
+  using profile = ti::EquatableNothrowMovableProfile;
+
+  {
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadSelfSwap)
+        .DUE_TO("Swap")
+        .INITIALIZER(BadSelfSwap())
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+struct BadDefaultInitializedMoveAssign {
+  BadDefaultInitializedMoveAssign() : default_initialized(true) {}
+  explicit BadDefaultInitializedMoveAssign(int v) : value(v) {}
+  BadDefaultInitializedMoveAssign(
+      BadDefaultInitializedMoveAssign&& other) noexcept
+      : value(other.value) {}
+  BadDefaultInitializedMoveAssign& operator=(
+      BadDefaultInitializedMoveAssign&& other) noexcept {
+    value = other.value;
+    if (default_initialized) ++value;  // Bad move if lhs is default initialized
+    return *this;
+  }
+
+  friend bool operator==(const BadDefaultInitializedMoveAssign& lhs,
+                         const BadDefaultInitializedMoveAssign& rhs) {
+    return lhs.value == rhs.value;
+  }
+
+  friend bool operator!=(const BadDefaultInitializedMoveAssign& lhs,
+                         const BadDefaultInitializedMoveAssign& rhs) {
+    return lhs.value != rhs.value;
+  }
+
+  bool default_initialized = false;
+  int value = 0;
+};
+
+TEST(ConformanceTestingDeathTest, DefaultInitializedMoveAssignFailure) {
+  using profile =
+      ti::CombineProfiles<ti::DefaultConstructibleNothrowMovableProfile,
+                          ti::EquatableProfile>;
+
+  {
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadDefaultInitializedMoveAssign)
+        .DUE_TO("move assignment")
+        .INITIALIZER(BadDefaultInitializedMoveAssign(0))
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
+
+struct BadDefaultInitializedCopyAssign {
+  BadDefaultInitializedCopyAssign() : default_initialized(true) {}
+  explicit BadDefaultInitializedCopyAssign(int v) : value(v) {}
+  BadDefaultInitializedCopyAssign(
+      BadDefaultInitializedCopyAssign&& other) noexcept
+      : value(other.value) {}
+  BadDefaultInitializedCopyAssign(const BadDefaultInitializedCopyAssign& other)
+      : value(other.value) {}
+
+  BadDefaultInitializedCopyAssign& operator=(
+      BadDefaultInitializedCopyAssign&& other) noexcept {
+    value = other.value;
+    return *this;
+  }
+
+  BadDefaultInitializedCopyAssign& operator=(
+      const BadDefaultInitializedCopyAssign& other) {
+    value = other.value;
+    if (default_initialized) ++value;  // Bad move if lhs is default initialized
+    return *this;
+  }
+
+  friend bool operator==(const BadDefaultInitializedCopyAssign& lhs,
+                         const BadDefaultInitializedCopyAssign& rhs) {
+    return lhs.value == rhs.value;
+  }
+
+  friend bool operator!=(const BadDefaultInitializedCopyAssign& lhs,
+                         const BadDefaultInitializedCopyAssign& rhs) {
+    return lhs.value != rhs.value;
+  }
+
+  bool default_initialized = false;
+  int value = 0;
+};
+
+TEST(ConformanceTestingDeathTest, DefaultInitializedAssignFailure) {
+  using profile = ti::CombineProfiles<ti::DefaultConstructibleValueProfile,
+                                      ti::EquatableProfile>;
+
+  {
+    ABSL_INTERNAL_ASSERT_NONCONFORMANCE_OF(BadDefaultInitializedCopyAssign)
+        .DUE_TO("copy assignment")
+        .INITIALIZER(BadDefaultInitializedCopyAssign(0))
+        .WITH_LOOSE_PROFILE(profile);
+  }
+}
 
 }  // namespace

diff --git a/absl/types/internal/parentheses.h b/absl/types/internal/parentheses.h
new file mode 100644
index 0000000..5aebee8
--- /dev/null
+++ b/absl/types/internal/parentheses.h

@@ -0,0 +1,34 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// parentheses.h
+// -----------------------------------------------------------------------------
+//
+// This file contains macros that expand to a left parenthesis and a right
+// parenthesis. These are in their own file and are generated from macros
+// because otherwise clang-format gets confused and clang-format off directives
+// do not help.
+//
+// The parentheses macros are used when wanting to require a rescan before
+// expansion of parenthesized text appearing after a function-style macro name.
+
+#ifndef ABSL_TYPES_INTERNAL_PARENTHESES_H_
+#define ABSL_TYPES_INTERNAL_PARENTHESES_H_
+
+#define ABSL_INTERNAL_LPAREN (
+
+#define ABSL_INTERNAL_RPAREN )
+
+#endif  // ABSL_TYPES_INTERNAL_PARENTHESES_H_

diff --git a/absl/types/internal/transform_args.h b/absl/types/internal/transform_args.h
new file mode 100644
index 0000000..4a0ab42
--- /dev/null
+++ b/absl/types/internal/transform_args.h

@@ -0,0 +1,246 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// transform_args.h
+// -----------------------------------------------------------------------------
+//
+// This file contains a higher-order macro that "transforms" each element of a
+// a variadic argument by a provided secondary macro.
+
+#ifndef ABSL_TYPES_INTERNAL_TRANSFORM_ARGS_H_
+#define ABSL_TYPES_INTERNAL_TRANSFORM_ARGS_H_
+
+//
+// ABSL_INTERNAL_CAT(a, b)
+//
+// This macro takes two arguments and concatenates them together via ## after
+// expansion.
+//
+// Example:
+//
+//   ABSL_INTERNAL_CAT(foo_, bar)
+//
+// Results in:
+//
+//   foo_bar
+#define ABSL_INTERNAL_CAT(a, b) ABSL_INTERNAL_CAT_IMPL(a, b)
+#define ABSL_INTERNAL_CAT_IMPL(a, b) a##b
+
+//
+// ABSL_INTERNAL_TRANSFORM_ARGS(m, ...)
+//
+// This macro takes another macro as an argument followed by a trailing series
+// of additional parameters (up to 32 additional arguments). It invokes the
+// passed-in macro once for each of the additional arguments, with the
+// expansions separated by commas.
+//
+// Example:
+//
+//   ABSL_INTERNAL_TRANSFORM_ARGS(MY_MACRO, a, b, c)
+//
+// Results in:
+//
+//   MY_MACRO(a), MY_MACRO(b), MY_MACRO(c)
+//
+// TODO(calabrese) Handle no arguments as a special case.
+#define ABSL_INTERNAL_TRANSFORM_ARGS(m, ...)             \
+  ABSL_INTERNAL_CAT(ABSL_INTERNAL_TRANSFORM_ARGS,        \
+                    ABSL_INTERNAL_NUM_ARGS(__VA_ARGS__)) \
+  (m, __VA_ARGS__)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS1(m, a0) m(a0)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS2(m, a0, a1) m(a0), m(a1)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS3(m, a0, a1, a2) m(a0), m(a1), m(a2)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS4(m, a0, a1, a2, a3) \
+  m(a0), m(a1), m(a2), m(a3)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS5(m, a0, a1, a2, a3, a4) \
+  m(a0), m(a1), m(a2), m(a3), m(a4)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS6(m, a0, a1, a2, a3, a4, a5) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS7(m, a0, a1, a2, a3, a4, a5, a6) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS8(m, a0, a1, a2, a3, a4, a5, a6, a7) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS9(m, a0, a1, a2, a3, a4, a5, a6, a7, a8) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS10(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9)                                    \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS11(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10)                               \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9), m(a10)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS12(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11)                          \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS13(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12)                     \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS14(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13)                \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS15(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14)           \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS16(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15)      \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS17(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS18(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17)                                   \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS19(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18)                              \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS20(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18, a19)                         \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS21(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18, a19, a20)                    \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS22(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18, a19, a20, a21)               \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS23(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18, a19, a20, a21, a22)          \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS24(m, a0, a1, a2, a3, a4, a5, a6, a7, a8, \
+                                       a9, a10, a11, a12, a13, a14, a15, a16, \
+                                       a17, a18, a19, a20, a21, a22, a23)     \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS25(m, a0, a1, a2, a3, a4, a5, a6, a7, a8,  \
+                                       a9, a10, a11, a12, a13, a14, a15, a16,  \
+                                       a17, a18, a19, a20, a21, a22, a23, a24) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),        \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18),  \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS26(                                       \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,  \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25)                         \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS27(                                       \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,  \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26)                    \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS28(                                       \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,  \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27)               \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26), m(a27)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS29(                                       \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,  \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27, a28)          \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26), m(a27), \
+      m(a28)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS30(                                       \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,  \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27, a28, a29)     \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),       \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18), \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26), m(a27), \
+      m(a28), m(a29)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS31(                                        \
+    m, a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,   \
+    a16, a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27, a28, a29, a30) \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),        \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18),  \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26), m(a27),  \
+      m(a28), m(a29), m(a30)
+
+#define ABSL_INTERNAL_TRANSFORM_ARGS32(m, a0, a1, a2, a3, a4, a5, a6, a7, a8,  \
+                                       a9, a10, a11, a12, a13, a14, a15, a16,  \
+                                       a17, a18, a19, a20, a21, a22, a23, a24, \
+                                       a25, a26, a27, a28, a29, a30, a31)      \
+  m(a0), m(a1), m(a2), m(a3), m(a4), m(a5), m(a6), m(a7), m(a8), m(a9),        \
+      m(a10), m(a11), m(a12), m(a13), m(a14), m(a15), m(a16), m(a17), m(a18),  \
+      m(a19), m(a20), m(a21), m(a22), m(a23), m(a24), m(a25), m(a26), m(a27),  \
+      m(a28), m(a29), m(a30), m(a31)
+
+#define ABSL_INTERNAL_NUM_ARGS_IMPL(a0, a1, a2, a3, a4, a5, a6, a7, a8, a9,    \
+                                    a10, a11, a12, a13, a14, a15, a16, a17,    \
+                                    a18, a19, a20, a21, a22, a23, a24, a25,    \
+                                    a26, a27, a28, a29, a30, a31, result, ...) \
+  result
+
+#define ABSL_INTERNAL_FORCE_EXPANSION(...) __VA_ARGS__
+
+#define ABSL_INTERNAL_NUM_ARGS(...)                                            \
+  ABSL_INTERNAL_FORCE_EXPANSION(ABSL_INTERNAL_NUM_ARGS_IMPL(                   \
+      __VA_ARGS__, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, \
+      17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, ))
+
+#endif  // ABSL_TYPES_INTERNAL_TRANSFORM_ARGS_H_

diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
index 874334e..7ef142c 100644
--- a/absl/types/optional_test.cc
+++ b/absl/types/optional_test.cc

@@ -1051,7 +1051,7 @@
 #ifdef ABSL_HAVE_EXCEPTIONS
   EXPECT_THROW((void)empty.value(), absl::bad_optional_access);
 #else
-  EXPECT_DEATH((void)empty.value(), "Bad optional access");
+  EXPECT_DEATH_IF_SUPPORTED((void)empty.value(), "Bad optional access");
 #endif
 
   // test constexpr value()

diff --git a/absl/types/variant_test.cc b/absl/types/variant_test.cc
index 4639c42..cf8f7f3 100644
--- a/absl/types/variant_test.cc
+++ b/absl/types/variant_test.cc

@@ -50,7 +50,7 @@
 #else
 
 #define ABSL_VARIANT_TEST_EXPECT_FAIL(expr, exception_t, text) \
-  EXPECT_DEATH(expr, text)
+  EXPECT_DEATH_IF_SUPPORTED(expr, text)
 
 #endif  // ABSL_HAVE_EXCEPTIONS
 

diff --git a/ci/cmake_install_test.sh b/ci/cmake_install_test.sh
index 4c748eb..b31e4b8 100755
--- a/ci/cmake_install_test.sh
+++ b/ci/cmake_install_test.sh

@@ -16,7 +16,7 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 

diff --git a/ci/linux_clang-latest_libcxx_asan_bazel.sh b/ci/linux_clang-latest_libcxx_asan_bazel.sh
index 03463e2..44794eb 100755
--- a/ci/linux_clang-latest_libcxx_asan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_asan_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++11 c++14 c++17 c++20"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi

diff --git a/ci/linux_clang-latest_libcxx_bazel.sh b/ci/linux_clang-latest_libcxx_bazel.sh
index 050a98c..eb04e69 100755
--- a/ci/linux_clang-latest_libcxx_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++11 c++14 c++17 c++20"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi

diff --git a/ci/linux_clang-latest_libcxx_tsan_bazel.sh b/ci/linux_clang-latest_libcxx_tsan_bazel.sh
index 2f5fb12..c2eb5ba 100755
--- a/ci/linux_clang-latest_libcxx_tsan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_tsan_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++11 c++14 c++17 c++20"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi

diff --git a/ci/linux_clang-latest_libstdcxx_bazel.sh b/ci/linux_clang-latest_libstdcxx_bazel.sh
index 087f59b..0192ee4 100755
--- a/ci/linux_clang-latest_libstdcxx_bazel.sh
+++ b/ci/linux_clang-latest_libstdcxx_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++11 c++14 c++17 c++20"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi
@@ -71,7 +71,7 @@
         -e CC="/opt/llvm/clang/bin/clang" \
         -e BAZEL_COMPILER="llvm" \
         -e BAZEL_CXXOPTS="-std=${std}" \
-        -e CPLUS_INCLUDE_PATH="/usr/include/c++/6" \
+        -e CPLUS_INCLUDE_PATH="/usr/include/c++/8" \
         ${DOCKER_EXTRA_ARGS:-} \
         ${DOCKER_CONTAINER} \
         /usr/local/bin/bazel test ... \

diff --git a/ci/linux_docker_containers.sh b/ci/linux_docker_containers.sh
index cf056b3..82a10ac 100644
--- a/ci/linux_docker_containers.sh
+++ b/ci/linux_docker_containers.sh

@@ -16,6 +16,6 @@
 # Test scripts should source this file to get the identifiers.
 
 readonly LINUX_ALPINE_CONTAINER="gcr.io/google.com/absl-177019/alpine:20191016"
-readonly LINUX_CLANG_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200319"
+readonly LINUX_CLANG_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200401"
 readonly LINUX_GCC_LATEST_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-latest:20200319"
 readonly LINUX_GCC_49_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-4.9:20191018"

diff --git a/ci/linux_gcc-4.9_libstdcxx_bazel.sh b/ci/linux_gcc-4.9_libstdcxx_bazel.sh
index 622ea84..8e6540c 100755
--- a/ci/linux_gcc-4.9_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-4.9_libstdcxx_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
+if [[ -z ${STD:-} ]]; then
   STD="c++11 c++14"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi

diff --git a/ci/linux_gcc-latest_libstdcxx_bazel.sh b/ci/linux_gcc-latest_libstdcxx_bazel.sh
index f7dbd19..3ec0226 100755
--- a/ci/linux_gcc-latest_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-latest_libstdcxx_bazel.sh

@@ -20,19 +20,19 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${STD:-} ]; then
-  STD="c++11 c++14 c++17"
+if [[ -z ${STD:-} ]]; then
+  STD="c++11 c++14 c++17 c++2a"
 fi
 
-if [ -z ${COMPILATION_MODE:-} ]; then
+if [[ -z ${COMPILATION_MODE:-} ]]; then
   COMPILATION_MODE="fastbuild opt"
 fi
 
-if [ -z ${EXCEPTIONS_MODE:-} ]; then
+if [[ -z ${EXCEPTIONS_MODE:-} ]]; then
   EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
 fi
 
@@ -41,7 +41,7 @@
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
-if [ ${USE_BAZEL_CACHE:-0} -ne 0 ]; then
+if [[ ${USE_BAZEL_CACHE:-0} -ne 0 ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_KEYSTORE_DIR}:/keystore:ro ${DOCKER_EXTRA_ARGS:-}"
   # Bazel doesn't track changes to tools outside of the workspace
   # (e.g. /usr/bin/gcc), so by appending the docker container to the
@@ -54,7 +54,7 @@
 # Avoid depending on external sites like GitHub by checking --distdir for
 # external dependencies first.
 # https://docs.bazel.build/versions/master/guide.html#distdir
-if [ -z ${KOKORO_GFILE_DIR:-} && -d "${KOKORO_GFILE_DIR}/distdir" ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -d "${KOKORO_GFILE_DIR}/distdir" ]]; then
   DOCKER_EXTRA_ARGS="--volume=${KOKORO_GFILE_DIR}/distdir:/distdir:ro ${DOCKER_EXTRA_ARGS:-}"
   BAZEL_EXTRA_ARGS="--distdir=/distdir ${BAZEL_EXTRA_ARGS:-}"
 fi
@@ -75,7 +75,7 @@
         ${DOCKER_CONTAINER} \
         /bin/sh -c "
           cp -r /abseil-cpp-ro/* /abseil-cpp/
-          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+          if [[ -n \"${ALTERNATE_OPTIONS:-}\" ]]; then
             cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
           fi
           /usr/local/bin/bazel test ... \

diff --git a/ci/linux_gcc-latest_libstdcxx_cmake.sh b/ci/linux_gcc-latest_libstdcxx_cmake.sh
index b501544..db5f691 100755
--- a/ci/linux_gcc-latest_libstdcxx_cmake.sh
+++ b/ci/linux_gcc-latest_libstdcxx_cmake.sh

@@ -22,15 +22,15 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]; then
+if [[ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]]; then
   ABSL_CMAKE_CXX_STANDARDS="11 14 17"
 fi
 
-if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
   ABSL_CMAKE_BUILD_TYPES="Debug Release"
 fi
 

diff --git a/ci/linux_gcc_alpine_cmake.sh b/ci/linux_gcc_alpine_cmake.sh
index 4496f8d..f57ab12 100755
--- a/ci/linux_gcc_alpine_cmake.sh
+++ b/ci/linux_gcc_alpine_cmake.sh

@@ -22,15 +22,15 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-if [ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]; then
+if [[ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]]; then
   ABSL_CMAKE_CXX_STANDARDS="11 14 17"
 fi
 
-if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
   ABSL_CMAKE_BUILD_TYPES="Debug Release"
 fi
 

diff --git a/ci/macos_xcode_bazel.sh b/ci/macos_xcode_bazel.sh
index f5f2d75..738adf9 100755
--- a/ci/macos_xcode_bazel.sh
+++ b/ci/macos_xcode_bazel.sh

@@ -19,13 +19,13 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
 # If we are running on Kokoro, check for a versioned Bazel binary.
 KOKORO_GFILE_BAZEL_BIN="bazel-2.0.0-darwin-x86_64"
-if [ ${KOKORO_GFILE_DIR:-} ] && [ -f ${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN} ]; then
+if [[ ${KOKORO_GFILE_DIR:-} ]] && [[ -f ${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN} ]]; then
   BAZEL_BIN="${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN}"
   chmod +x ${BAZEL_BIN}
 else
@@ -41,7 +41,7 @@
 
 cd ${ABSEIL_ROOT}
 
-if [ -n "${ALTERNATE_OPTIONS:-}" ]; then
+if [[ -n "${ALTERNATE_OPTIONS:-}" ]]; then
   cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
 fi
 

diff --git a/ci/macos_xcode_cmake.sh b/ci/macos_xcode_cmake.sh
index a1f4a85..cf78e20 100755
--- a/ci/macos_xcode_cmake.sh
+++ b/ci/macos_xcode_cmake.sh

@@ -19,12 +19,12 @@
 
 set -euox pipefail
 
-if [ -z ${ABSEIL_ROOT:-} ]; then
+if [[ -z ${ABSEIL_ROOT:-} ]]; then
   ABSEIL_ROOT="$(dirname ${0})/.."
 fi
 ABSEIL_ROOT=$(realpath ${ABSEIL_ROOT})
 
-if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
+if [[ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]]; then
   ABSL_CMAKE_BUILD_TYPES="Debug"
 fi