diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 07ae20f..e25174a 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake

@@ -137,11 +137,12 @@
   "random/gaussian_distribution.cc"
   "random/gaussian_distribution.h"
   "random/internal/distribution_caller.h"
-  "random/internal/fast_uniform_bits.h"
   "random/internal/fastmath.h"
+  "random/internal/fast_uniform_bits.h"
   "random/internal/gaussian_distribution_gentables.cc"
   "random/internal/generate_real.h"
   "random/internal/iostream_state_saver.h"
+  "random/internal/mock_helpers.h"
   "random/internal/nonsecure_base.h"
   "random/internal/pcg_engine.h"
   "random/internal/platform.h"

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 35cb2e9..745a598 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel

@@ -799,3 +799,16 @@
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_test(
+    name = "optimization_test",
+    size = "small",
+    srcs = ["optimization_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":core_headers",
+        "//absl/types:optional",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index b7c01c9..62486f9 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt

@@ -701,3 +701,16 @@
     absl::fast_type_id
     gtest_main
 )
+
+absl_cc_test(
+  NAME
+    optimization_test
+  SRCS
+    "optimization_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::optional
+    gtest_main
+)

diff --git a/absl/base/call_once.h b/absl/base/call_once.h
index bc5ec93..5b468af 100644
--- a/absl/base/call_once.h
+++ b/absl/base/call_once.h

@@ -175,7 +175,7 @@
                                        std::memory_order_relaxed) ||
       base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans,
                                   scheduling_mode) == kOnceInit) {
-    base_internal::Invoke(std::forward<Callable>(fn),
+    base_internal::invoke(std::forward<Callable>(fn),
                           std::forward<Args>(args)...);
     // The call to SpinLockWake below is an optimization, because the waiter
     // in SpinLockWait is waiting with a short timeout. The atomic load/store

diff --git a/absl/base/internal/invoke.h b/absl/base/internal/invoke.h
index c4eceeb..5c71f32 100644
--- a/absl/base/internal/invoke.h
+++ b/absl/base/internal/invoke.h

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //
-// absl::base_internal::Invoke(f, args...) is an implementation of
+// absl::base_internal::invoke(f, args...) is an implementation of
 // INVOKE(f, args...) from section [func.require] of the C++ standard.
 //
 // [func.require]
@@ -29,7 +29,7 @@
 //    is not one of the types described in the previous item;
 // 5. f(t1, t2, ..., tN) in all other cases.
 //
-// The implementation is SFINAE-friendly: substitution failure within Invoke()
+// The implementation is SFINAE-friendly: substitution failure within invoke()
 // isn't an error.
 
 #ifndef ABSL_BASE_INTERNAL_INVOKE_H_
@@ -170,13 +170,13 @@
 
 // The result type of Invoke<F, Args...>.
 template <typename F, typename... Args>
-using InvokeT = decltype(Invoker<F, Args...>::type::Invoke(
+using invoke_result_t = decltype(Invoker<F, Args...>::type::Invoke(
     std::declval<F>(), std::declval<Args>()...));
 
 // Invoke(f, args...) is an implementation of INVOKE(f, args...) from section
 // [func.require] of the C++ standard.
 template <typename F, typename... Args>
-InvokeT<F, Args...> Invoke(F&& f, Args&&... args) {
+invoke_result_t<F, Args...> invoke(F&& f, Args&&... args) {
   return Invoker<F, Args...>::type::Invoke(std::forward<F>(f),
                                            std::forward<Args>(args)...);
 }

diff --git a/absl/base/invoke_test.cc b/absl/base/invoke_test.cc
index 6aa613c..bcdef36 100644
--- a/absl/base/invoke_test.cc
+++ b/absl/base/invoke_test.cc

@@ -86,71 +86,73 @@
   int member;
 };
 
-// CallMaybeWithArg(f) resolves either to Invoke(f) or Invoke(f, 42), depending
+// CallMaybeWithArg(f) resolves either to invoke(f) or invoke(f, 42), depending
 // on which one is valid.
 template <typename F>
-decltype(Invoke(std::declval<const F&>())) CallMaybeWithArg(const F& f) {
-  return Invoke(f);
+decltype(base_internal::invoke(std::declval<const F&>())) CallMaybeWithArg(
+    const F& f) {
+  return base_internal::invoke(f);
 }
 
 template <typename F>
-decltype(Invoke(std::declval<const F&>(), 42)) CallMaybeWithArg(const F& f) {
-  return Invoke(f, 42);
+decltype(base_internal::invoke(std::declval<const F&>(), 42)) CallMaybeWithArg(
+    const F& f) {
+  return base_internal::invoke(f, 42);
 }
 
 TEST(InvokeTest, Function) {
-  EXPECT_EQ(1, Invoke(Function, 3, 2));
-  EXPECT_EQ(1, Invoke(&Function, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(Function, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Function, 3, 2));
 }
 
 TEST(InvokeTest, NonCopyableArgument) {
-  EXPECT_EQ(42, Invoke(Sink, make_unique<int>(42)));
+  EXPECT_EQ(42, base_internal::invoke(Sink, make_unique<int>(42)));
 }
 
 TEST(InvokeTest, NonCopyableResult) {
-  EXPECT_THAT(Invoke(Factory, 42), ::testing::Pointee(42));
+  EXPECT_THAT(base_internal::invoke(Factory, 42), ::testing::Pointee(42));
 }
 
-TEST(InvokeTest, VoidResult) {
-  Invoke(NoOp);
-}
+TEST(InvokeTest, VoidResult) { base_internal::invoke(NoOp); }
 
 TEST(InvokeTest, ConstFunctor) {
-  EXPECT_EQ(1, Invoke(ConstFunctor(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(ConstFunctor(), 3, 2));
 }
 
 TEST(InvokeTest, MutableFunctor) {
   MutableFunctor f;
-  EXPECT_EQ(1, Invoke(f, 3, 2));
-  EXPECT_EQ(1, Invoke(MutableFunctor(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(f, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(MutableFunctor(), 3, 2));
 }
 
 TEST(InvokeTest, EphemeralFunctor) {
   EphemeralFunctor f;
-  EXPECT_EQ(1, Invoke(std::move(f), 3, 2));
-  EXPECT_EQ(1, Invoke(EphemeralFunctor(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(std::move(f), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(EphemeralFunctor(), 3, 2));
 }
 
 TEST(InvokeTest, OverloadedFunctor) {
   OverloadedFunctor f;
   const OverloadedFunctor& cf = f;
 
-  EXPECT_EQ("&", Invoke(f));
-  EXPECT_EQ("& 42", Invoke(f, " 42"));
+  EXPECT_EQ("&", base_internal::invoke(f));
+  EXPECT_EQ("& 42", base_internal::invoke(f, " 42"));
 
-  EXPECT_EQ("const&", Invoke(cf));
-  EXPECT_EQ("const& 42", Invoke(cf, " 42"));
+  EXPECT_EQ("const&", base_internal::invoke(cf));
+  EXPECT_EQ("const& 42", base_internal::invoke(cf, " 42"));
 
-  EXPECT_EQ("&&", Invoke(std::move(f)));
-  EXPECT_EQ("&& 42", Invoke(std::move(f), " 42"));
+  EXPECT_EQ("&&", base_internal::invoke(std::move(f)));
+
+  OverloadedFunctor f2;
+  EXPECT_EQ("&& 42", base_internal::invoke(std::move(f2), " 42"));
 }
 
 TEST(InvokeTest, ReferenceWrapper) {
   ConstFunctor cf;
   MutableFunctor mf;
-  EXPECT_EQ(1, Invoke(std::cref(cf), 3, 2));
-  EXPECT_EQ(1, Invoke(std::ref(cf), 3, 2));
-  EXPECT_EQ(1, Invoke(std::ref(mf), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(std::cref(cf), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(std::ref(cf), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(std::ref(mf), 3, 2));
 }
 
 TEST(InvokeTest, MemberFunction) {
@@ -158,58 +160,62 @@
   std::unique_ptr<const Class> cp(new Class);
   std::unique_ptr<volatile Class> vp(new Class);
 
-  EXPECT_EQ(1, Invoke(&Class::Method, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::Method, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::Method, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefMethod, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::RefRefMethod, std::move(*p), 3, 2));  // NOLINT
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::NoExceptMethod, *p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::Method, p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::Method, p.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::Method, *p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::RefMethod, p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::RefMethod, p.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::RefMethod, *p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::RefRefMethod, std::move(*p), 3,
+                                     2));  // NOLINT
+  EXPECT_EQ(1, base_internal::invoke(&Class::NoExceptMethod, p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::NoExceptMethod, p.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::NoExceptMethod, *p, 3, 2));
 
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, *p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, p.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, *p, 3, 2));
 
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, cp, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, cp.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, *cp, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, cp, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, cp.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, *cp, 3, 2));
 
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, p.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, *p, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, vp, 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, vp.get(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::VolatileMethod, *vp, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, p.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, *p, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, vp, 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, vp.get(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::VolatileMethod, *vp, 3, 2));
 
-  EXPECT_EQ(1, Invoke(&Class::Method, make_unique<Class>(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, make_unique<Class>(), 3, 2));
-  EXPECT_EQ(1, Invoke(&Class::ConstMethod, make_unique<const Class>(), 3, 2));
+  EXPECT_EQ(1,
+            base_internal::invoke(&Class::Method, make_unique<Class>(), 3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod, make_unique<Class>(),
+                                     3, 2));
+  EXPECT_EQ(1, base_internal::invoke(&Class::ConstMethod,
+                                     make_unique<const Class>(), 3, 2));
 }
 
 TEST(InvokeTest, DataMember) {
   std::unique_ptr<Class> p(new Class{42});
   std::unique_ptr<const Class> cp(new Class{42});
-  EXPECT_EQ(42, Invoke(&Class::member, p));
-  EXPECT_EQ(42, Invoke(&Class::member, *p));
-  EXPECT_EQ(42, Invoke(&Class::member, p.get()));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, p));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, *p));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, p.get()));
 
-  Invoke(&Class::member, p) = 42;
-  Invoke(&Class::member, p.get()) = 42;
+  base_internal::invoke(&Class::member, p) = 42;
+  base_internal::invoke(&Class::member, p.get()) = 42;
 
-  EXPECT_EQ(42, Invoke(&Class::member, cp));
-  EXPECT_EQ(42, Invoke(&Class::member, *cp));
-  EXPECT_EQ(42, Invoke(&Class::member, cp.get()));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, cp));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, *cp));
+  EXPECT_EQ(42, base_internal::invoke(&Class::member, cp.get()));
 }
 
 TEST(InvokeTest, FlipFlop) {
   FlipFlop obj = {42};
   // This call could resolve to (obj.*&FlipFlop::ConstMethod)() or
   // ((*obj).*&FlipFlop::ConstMethod)(). We verify that it's the former.
-  EXPECT_EQ(42, Invoke(&FlipFlop::ConstMethod, obj));
-  EXPECT_EQ(42, Invoke(&FlipFlop::member, obj));
+  EXPECT_EQ(42, base_internal::invoke(&FlipFlop::ConstMethod, obj));
+  EXPECT_EQ(42, base_internal::invoke(&FlipFlop::member, obj));
 }
 
 TEST(InvokeTest, SfinaeFriendly) {

diff --git a/absl/base/optimization_test.cc b/absl/base/optimization_test.cc
new file mode 100644
index 0000000..894b68f
--- /dev/null
+++ b/absl/base/optimization_test.cc

@@ -0,0 +1,132 @@
+// Copyright 2020 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.
+
+#include "absl/base/optimization.h"
+
+#include "gtest/gtest.h"
+#include "absl/types/optional.h"
+
+namespace {
+
+// Tests for the ABSL_PREDICT_TRUE and ABSL_PREDICT_FALSE macros.
+// The tests only verify that the macros are functionally correct - i.e. code
+// behaves as if they weren't used. They don't try to check their impact on
+// optimization.
+
+TEST(PredictTest, PredictTrue) {
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(false));
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(1 == 1));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(1 == 2));
+
+  if (ABSL_PREDICT_TRUE(false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_TRUE(true)) ADD_FAILURE();
+
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true) && true);
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(true) || false);
+}
+
+TEST(PredictTest, PredictFalse) {
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(false));
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(1 == 1));
+  EXPECT_FALSE(ABSL_PREDICT_FALSE(1 == 2));
+
+  if (ABSL_PREDICT_FALSE(false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_FALSE(true)) ADD_FAILURE();
+
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true) && true);
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(true) || false);
+}
+
+TEST(PredictTest, OneEvaluation) {
+  // Verify that the expression is only evaluated once.
+  int x = 0;
+  if (ABSL_PREDICT_TRUE((++x) == 0)) ADD_FAILURE();
+  EXPECT_EQ(x, 1);
+  if (ABSL_PREDICT_FALSE((++x) == 0)) ADD_FAILURE();
+  EXPECT_EQ(x, 2);
+}
+
+TEST(PredictTest, OperatorOrder) {
+  // Verify that operator order inside and outside the macro behaves well.
+  // These would fail for a naive '#define ABSL_PREDICT_TRUE(x) x'
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(1 && 2) == true);
+  EXPECT_TRUE(ABSL_PREDICT_FALSE(1 && 2) == true);
+  EXPECT_TRUE(!ABSL_PREDICT_TRUE(1 == 2));
+  EXPECT_TRUE(!ABSL_PREDICT_FALSE(1 == 2));
+}
+
+TEST(PredictTest, Pointer) {
+  const int x = 3;
+  const int *good_intptr = &x;
+  const int *null_intptr = nullptr;
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(good_intptr));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(null_intptr));
+  // The following doesn't compile:
+  // EXPECT_TRUE(ABSL_PREDICT_FALSE(good_intptr));
+  // EXPECT_FALSE(ABSL_PREDICT_FALSE(null_intptr));
+}
+
+TEST(PredictTest, Optional) {
+  // Note: An optional's truth value is the value's existence, not its truth.
+  absl::optional<bool> has_value(false);
+  absl::optional<bool> no_value;
+  EXPECT_TRUE(ABSL_PREDICT_TRUE(has_value));
+  EXPECT_FALSE(ABSL_PREDICT_TRUE(no_value));
+  // The following doesn't compile:
+  // EXPECT_TRUE(ABSL_PREDICT_FALSE(has_value));
+  // EXPECT_FALSE(ABSL_PREDICT_FALSE(no_value));
+}
+
+class ImplictlyConvertibleToBool {
+ public:
+  explicit ImplictlyConvertibleToBool(bool value) : value_(value) {}
+  operator bool() const {  // NOLINT(google-explicit-constructor)
+    return value_;
+  }
+
+ private:
+  bool value_;
+};
+
+TEST(PredictTest, ImplicitBoolConversion) {
+  const ImplictlyConvertibleToBool is_true(true);
+  const ImplictlyConvertibleToBool is_false(false);
+  if (!ABSL_PREDICT_TRUE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_TRUE(is_false)) ADD_FAILURE();
+  if (!ABSL_PREDICT_FALSE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_FALSE(is_false)) ADD_FAILURE();
+}
+
+class ExplictlyConvertibleToBool {
+ public:
+  explicit ExplictlyConvertibleToBool(bool value) : value_(value) {}
+  explicit operator bool() const { return value_; }
+
+ private:
+  bool value_;
+};
+
+TEST(PredictTest, ExplicitBoolConversion) {
+  const ExplictlyConvertibleToBool is_true(true);
+  const ExplictlyConvertibleToBool is_false(false);
+  if (!ABSL_PREDICT_TRUE(is_true)) ADD_FAILURE();
+  if (ABSL_PREDICT_TRUE(is_false)) ADD_FAILURE();
+  // The following doesn't compile:
+  // if (!ABSL_PREDICT_FALSE(is_true)) ADD_FAILURE();
+  // if (ABSL_PREDICT_FALSE(is_false)) ADD_FAILURE();
+}
+
+}  // namespace

diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
index bbdb5f4..1738d29 100644
--- a/absl/container/btree_test.cc
+++ b/absl/container/btree_test.cc

@@ -52,6 +52,7 @@
 using ::testing::ElementsAre;
 using ::testing::ElementsAreArray;
 using ::testing::IsEmpty;
+using ::testing::IsNull;
 using ::testing::Pair;
 
 template <typename T, typename U>
@@ -2404,6 +2405,109 @@
   m[n];
 }
 
+TEST(Btree, SetRangeConstructorAndInsertSupportExplicitConversionComparable) {
+  const absl::string_view names[] = {"n1", "n2"};
+
+  absl::btree_set<std::string> name_set1{std::begin(names), std::end(names)};
+  EXPECT_THAT(name_set1, ElementsAreArray(names));
+
+  absl::btree_set<std::string> name_set2;
+  name_set2.insert(std::begin(names), std::end(names));
+  EXPECT_THAT(name_set2, ElementsAreArray(names));
+}
+
+// A type that is explicitly convertible from int and counts constructor calls.
+struct ConstructorCounted {
+  explicit ConstructorCounted(int i) : i(i) { ++constructor_calls; }
+  bool operator==(int other) const { return i == other; }
+
+  int i;
+  static int constructor_calls;
+};
+int ConstructorCounted::constructor_calls = 0;
+
+struct ConstructorCountedCompare {
+  bool operator()(int a, const ConstructorCounted &b) const { return a < b.i; }
+  bool operator()(const ConstructorCounted &a, int b) const { return a.i < b; }
+  bool operator()(const ConstructorCounted &a,
+                  const ConstructorCounted &b) const {
+    return a.i < b.i;
+  }
+  using is_transparent = void;
+};
+
+TEST(Btree,
+     SetRangeConstructorAndInsertExplicitConvComparableLimitConstruction) {
+  const int i[] = {0, 1, 1};
+  ConstructorCounted::constructor_calls = 0;
+
+  absl::btree_set<ConstructorCounted, ConstructorCountedCompare> set{
+      std::begin(i), std::end(i)};
+  EXPECT_THAT(set, ElementsAre(0, 1));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+
+  set.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(set, ElementsAre(0, 1));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+}
+
+TEST(Btree,
+     SetRangeConstructorAndInsertSupportExplicitConversionNonComparable) {
+  const int i[] = {0, 1};
+
+  absl::btree_set<std::vector<void *>> s1{std::begin(i), std::end(i)};
+  EXPECT_THAT(s1, ElementsAre(IsEmpty(), ElementsAre(IsNull())));
+
+  absl::btree_set<std::vector<void *>> s2;
+  s2.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(s2, ElementsAre(IsEmpty(), ElementsAre(IsNull())));
+}
+
+// GCC 4.9 has a bug in the std::pair constructors that prevents explicit
+// conversions between pair types.
+#if defined(__clang__) || !defined(__GNUC__) || __GNUC__ >= 5
+TEST(Btree, MapRangeConstructorAndInsertSupportExplicitConversionComparable) {
+  const std::pair<absl::string_view, int> names[] = {{"n1", 1}, {"n2", 2}};
+
+  absl::btree_map<std::string, int> name_map1{std::begin(names),
+                                              std::end(names)};
+  EXPECT_THAT(name_map1, ElementsAre(Pair("n1", 1), Pair("n2", 2)));
+
+  absl::btree_map<std::string, int> name_map2;
+  name_map2.insert(std::begin(names), std::end(names));
+  EXPECT_THAT(name_map2, ElementsAre(Pair("n1", 1), Pair("n2", 2)));
+}
+
+TEST(Btree,
+     MapRangeConstructorAndInsertExplicitConvComparableLimitConstruction) {
+  const std::pair<int, int> i[] = {{0, 1}, {1, 2}, {1, 3}};
+  ConstructorCounted::constructor_calls = 0;
+
+  absl::btree_map<ConstructorCounted, int, ConstructorCountedCompare> map{
+      std::begin(i), std::end(i)};
+  EXPECT_THAT(map, ElementsAre(Pair(0, 1), Pair(1, 2)));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+
+  map.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(map, ElementsAre(Pair(0, 1), Pair(1, 2)));
+  EXPECT_EQ(ConstructorCounted::constructor_calls, 2);
+}
+
+TEST(Btree,
+     MapRangeConstructorAndInsertSupportExplicitConversionNonComparable) {
+  const std::pair<int, int> i[] = {{0, 1}, {1, 2}};
+
+  absl::btree_map<std::vector<void *>, int> m1{std::begin(i), std::end(i)};
+  EXPECT_THAT(m1,
+              ElementsAre(Pair(IsEmpty(), 1), Pair(ElementsAre(IsNull()), 2)));
+
+  absl::btree_map<std::vector<void *>, int> m2;
+  m2.insert(std::begin(i), std::end(i));
+  EXPECT_THAT(m2,
+              ElementsAre(Pair(IsEmpty(), 1), Pair(ElementsAre(IsNull()), 2)));
+}
+#endif
+
 }  // namespace
 }  // namespace container_internal
 ABSL_NAMESPACE_END

diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index f52fe23..996afa6 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h

@@ -290,9 +290,12 @@
   };
   using is_map_container = std::true_type;
 
-  static const Key &key(const value_type &value) { return value.first; }
-  static const Key &key(const init_type &init) { return init.first; }
+  template <typename V>
+  static auto key(const V &value) -> decltype(value.first) {
+    return value.first;
+  }
   static const Key &key(const slot_type *s) { return slot_policy::key(s); }
+  static const Key &key(slot_type *s) { return slot_policy::key(s); }
   static mapped_type &value(value_type *value) { return value->second; }
 };
 
@@ -353,8 +356,10 @@
   using value_compare = typename set_params::common_params::key_compare;
   using is_map_container = std::false_type;
 
-  static const Key &key(const value_type &value) { return value; }
+  template <typename V>
+  static const V &key(const V &value) { return value; }
   static const Key &key(const slot_type *slot) { return *slot; }
+  static const Key &key(slot_type *slot) { return *slot; }
 };
 
 // An adapter class that converts a lower-bound compare into an upper-bound
@@ -1020,6 +1025,7 @@
 class btree {
   using node_type = btree_node<Params>;
   using is_key_compare_to = typename Params::is_key_compare_to;
+  using init_type = typename Params::init_type;
 
   // We use a static empty node for the root/leftmost/rightmost of empty btrees
   // in order to avoid branching in begin()/end().
@@ -1184,8 +1190,8 @@
   // boolean return value indicates whether insertion succeeded or failed.
   // Requirement: if `key` already exists in the btree, does not consume `args`.
   // Requirement: `key` is never referenced after consuming `args`.
-  template <typename... Args>
-  std::pair<iterator, bool> insert_unique(const key_type &key, Args &&... args);
+  template <typename K, typename... Args>
+  std::pair<iterator, bool> insert_unique(const K &key, Args &&... args);
 
   // Inserts with hint. Checks to see if the value should be placed immediately
   // before `position` in the tree. If so, then the insertion will take
@@ -1193,14 +1199,23 @@
   // logarithmic time as if a call to insert_unique() were made.
   // Requirement: if `key` already exists in the btree, does not consume `args`.
   // Requirement: `key` is never referenced after consuming `args`.
-  template <typename... Args>
+  template <typename K, typename... Args>
   std::pair<iterator, bool> insert_hint_unique(iterator position,
-                                               const key_type &key,
+                                               const K &key,
                                                Args &&... args);
 
   // Insert a range of values into the btree.
+  // Note: the first overload avoids constructing a value_type if the key
+  // already exists in the btree.
+  template <typename InputIterator,
+            typename = decltype(std::declval<const key_compare &>()(
+                params_type::key(*std::declval<InputIterator>()),
+                std::declval<const key_type &>()))>
+  void insert_iterator_unique(InputIterator b, InputIterator e, int);
+  // We need the second overload for cases in which we need to construct a
+  // value_type in order to compare it with the keys already in the btree.
   template <typename InputIterator>
-  void insert_iterator_unique(InputIterator b, InputIterator e);
+  void insert_iterator_unique(InputIterator b, InputIterator e, char);
 
   // Inserts a value into the btree.
   template <typename ValueType>
@@ -1855,8 +1870,8 @@
 }
 
 template <typename P>
-template <typename... Args>
-auto btree<P>::insert_unique(const key_type &key, Args &&... args)
+template <typename K, typename... Args>
+auto btree<P>::insert_unique(const K &key, Args &&... args)
     -> std::pair<iterator, bool> {
   if (empty()) {
     mutable_root() = rightmost_ = new_leaf_root_node(1);
@@ -1881,8 +1896,8 @@
 }
 
 template <typename P>
-template <typename... Args>
-inline auto btree<P>::insert_hint_unique(iterator position, const key_type &key,
+template <typename K, typename... Args>
+inline auto btree<P>::insert_hint_unique(iterator position, const K &key,
                                          Args &&... args)
     -> std::pair<iterator, bool> {
   if (!empty()) {
@@ -1906,14 +1921,23 @@
 }
 
 template <typename P>
-template <typename InputIterator>
-void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e) {
+template <typename InputIterator, typename>
+void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, int) {
   for (; b != e; ++b) {
     insert_hint_unique(end(), params_type::key(*b), *b);
   }
 }
 
 template <typename P>
+template <typename InputIterator>
+void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e, char) {
+  for (; b != e; ++b) {
+    init_type value(*b);
+    insert_hint_unique(end(), params_type::key(value), std::move(value));
+  }
+}
+
+template <typename P>
 template <typename ValueType>
 auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
   if (empty()) {

diff --git a/absl/container/internal/btree_container.h b/absl/container/internal/btree_container.h
index 734c90e..d0fc645 100644
--- a/absl/container/internal/btree_container.h
+++ b/absl/container/internal/btree_container.h

@@ -289,10 +289,10 @@
   }
   template <typename InputIterator>
   void insert(InputIterator b, InputIterator e) {
-    this->tree_.insert_iterator_unique(b, e);
+    this->tree_.insert_iterator_unique(b, e, 0);
   }
   void insert(std::initializer_list<init_type> init) {
-    this->tree_.insert_iterator_unique(init.begin(), init.end());
+    this->tree_.insert_iterator_unique(init.begin(), init.end(), 0);
   }
   insert_return_type insert(node_type &&node) {
     if (!node) return {this->end(), false, node_type()};

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index df0f2b2..1f304b6 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h

@@ -497,6 +497,18 @@
   return growth + static_cast<size_t>((static_cast<int64_t>(growth) - 1) / 7);
 }
 
+inline void AssertIsFull(ctrl_t* ctrl) {
+  ABSL_HARDENING_ASSERT((ctrl != nullptr && IsFull(*ctrl)) &&
+                        "Invalid operation on iterator. The element might have "
+                        "been erased, or the table might have rehashed.");
+}
+
+inline void AssertIsValid(ctrl_t* ctrl) {
+  ABSL_HARDENING_ASSERT((ctrl == nullptr || IsFull(*ctrl)) &&
+                        "Invalid operation on iterator. The element might have "
+                        "been erased, or the table might have rehashed.");
+}
+
 // Policy: a policy defines how to perform different operations on
 // the slots of the hashtable (see hash_policy_traits.h for the full interface
 // of policy).
@@ -617,7 +629,7 @@
 
     // PRECONDITION: not an end() iterator.
     reference operator*() const {
-      assert_is_full();
+      AssertIsFull(ctrl_);
       return PolicyTraits::element(slot_);
     }
 
@@ -626,7 +638,7 @@
 
     // PRECONDITION: not an end() iterator.
     iterator& operator++() {
-      assert_is_full();
+      AssertIsFull(ctrl_);
       ++ctrl_;
       ++slot_;
       skip_empty_or_deleted();
@@ -640,8 +652,8 @@
     }
 
     friend bool operator==(const iterator& a, const iterator& b) {
-      a.assert_is_valid();
-      b.assert_is_valid();
+      AssertIsValid(a.ctrl_);
+      AssertIsValid(b.ctrl_);
       return a.ctrl_ == b.ctrl_;
     }
     friend bool operator!=(const iterator& a, const iterator& b) {
@@ -655,13 +667,6 @@
       ABSL_INTERNAL_ASSUME(ctrl != nullptr);
     }
 
-    void assert_is_full() const {
-      ABSL_HARDENING_ASSERT(ctrl_ != nullptr && IsFull(*ctrl_));
-    }
-    void assert_is_valid() const {
-      ABSL_HARDENING_ASSERT(ctrl_ == nullptr || IsFull(*ctrl_));
-    }
-
     void skip_empty_or_deleted() {
       while (IsEmptyOrDeleted(*ctrl_)) {
         uint32_t shift = Group{ctrl_}.CountLeadingEmptyOrDeleted();
@@ -1174,7 +1179,7 @@
   // This overload is necessary because otherwise erase<K>(const K&) would be
   // a better match if non-const iterator is passed as an argument.
   void erase(iterator it) {
-    it.assert_is_full();
+    AssertIsFull(it.ctrl_);
     PolicyTraits::destroy(&alloc_ref(), it.slot_);
     erase_meta_only(it);
   }
@@ -1208,7 +1213,7 @@
   }
 
   node_type extract(const_iterator position) {
-    position.inner_.assert_is_full();
+    AssertIsFull(position.inner_.ctrl_);
     auto node =
         CommonAccess::Transfer<node_type>(alloc_ref(), position.inner_.slot_);
     erase_meta_only(position);

diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
index 2fc8559..6befa96 100644
--- a/absl/container/internal/raw_hash_set_test.cc
+++ b/absl/container/internal/raw_hash_set_test.cc

@@ -1791,7 +1791,7 @@
 
   IntTable t;
   // Extra simple "regexp" as regexp support is highly varied across platforms.
-  constexpr char kDeathMsg[] = "IsFull";
+  constexpr char kDeathMsg[] = "Invalid operation on iterator";
   EXPECT_DEATH_IF_SUPPORTED(t.erase(t.end()), kDeathMsg);
 }
 

diff --git a/absl/debugging/BUILD.bazel b/absl/debugging/BUILD.bazel
index ff62792..d336246 100644
--- a/absl/debugging/BUILD.bazel
+++ b/absl/debugging/BUILD.bazel

@@ -100,6 +100,7 @@
         "//absl/base:core_headers",
         "//absl/base:raw_logging_internal",
         "//absl/memory",
+        "//absl/strings",
         "@com_google_googletest//:gtest",
     ],
 )

diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index 995e888..c597df8 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt

@@ -85,6 +85,7 @@
     absl::core_headers
     absl::memory
     absl::raw_logging_internal
+    absl::strings
     gmock
 )
 

diff --git a/absl/debugging/internal/symbolize.h b/absl/debugging/internal/symbolize.h
index 8789e69..663d774 100644
--- a/absl/debugging/internal/symbolize.h
+++ b/absl/debugging/internal/symbolize.h

@@ -22,6 +22,7 @@
 #include <cstdint>
 
 #include "absl/base/config.h"
+#include "absl/strings/string_view.h"
 
 #ifdef ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
 #error ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE cannot be directly set
@@ -45,7 +46,7 @@
 //
 // This is not async-signal-safe.
 bool ForEachSection(int fd,
-                    const std::function<bool(const std::string& name,
+                    const std::function<bool(absl::string_view name,
                                              const ElfW(Shdr) &)>& callback);
 
 // Gets the section header for the given name, if it exists. Returns true on

diff --git a/absl/debugging/symbolize_elf.inc b/absl/debugging/symbolize_elf.inc
index ec86f9a..c05424e 100644
--- a/absl/debugging/symbolize_elf.inc
+++ b/absl/debugging/symbolize_elf.inc

@@ -74,6 +74,7 @@
 #include "absl/base/port.h"
 #include "absl/debugging/internal/demangle.h"
 #include "absl/debugging/internal/vdso_support.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -498,7 +499,7 @@
 const int kMaxSectionNameLen = 64;
 
 bool ForEachSection(int fd,
-                    const std::function<bool(const std::string &name,
+                    const std::function<bool(absl::string_view name,
                                              const ElfW(Shdr) &)> &callback) {
   ElfW(Ehdr) elf_header;
   if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
@@ -520,7 +521,7 @@
       return false;
     }
     off_t name_offset = shstrtab.sh_offset + out.sh_name;
-    char header_name[kMaxSectionNameLen + 1];
+    char header_name[kMaxSectionNameLen];
     ssize_t n_read =
         ReadFromOffset(fd, &header_name, kMaxSectionNameLen, name_offset);
     if (n_read == -1) {
@@ -529,9 +530,8 @@
       // Long read?
       return false;
     }
-    header_name[n_read] = '\0';
 
-    std::string name(header_name);
+    absl::string_view name(header_name, strnlen(header_name, n_read));
     if (!callback(name, out)) {
       break;
     }

diff --git a/absl/debugging/symbolize_test.cc b/absl/debugging/symbolize_test.cc
index e476d82..43f6554 100644
--- a/absl/debugging/symbolize_test.cc
+++ b/absl/debugging/symbolize_test.cc

@@ -32,6 +32,7 @@
 #include "absl/base/optimization.h"
 #include "absl/debugging/internal/stack_consumption.h"
 #include "absl/memory/memory.h"
+#include "absl/strings/string_view.h"
 
 using testing::Contains;
 
@@ -401,8 +402,8 @@
 
   std::vector<std::string> sections;
   ASSERT_TRUE(absl::debugging_internal::ForEachSection(
-      fd, [&sections](const std::string &name, const ElfW(Shdr) &) {
-        sections.push_back(name);
+      fd, [&sections](const absl::string_view name, const ElfW(Shdr) &) {
+        sections.emplace_back(name);
         return true;
       }));
 

diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 37c9a10..006911f 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel

@@ -169,9 +169,6 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    visibility = [
-        "//absl/flags:__pkg__",
-    ],
     deps = [
         ":commandlineflag",
         ":commandlineflag_internal",

diff --git a/absl/flags/internal/registry.h b/absl/flags/internal/registry.h
index c72eebe..6f5006a 100644
--- a/absl/flags/internal/registry.h
+++ b/absl/flags/internal/registry.h

@@ -28,15 +28,8 @@
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
-
-// TODO(rogeeff): remove this declaration
-CommandLineFlag* FindCommandLineFlag(absl::string_view name);
-
 namespace flags_internal {
 
-// TODO(rogeeff): remove this alias
-using absl::FindCommandLineFlag;
-
 // Executes specified visitor for each non-retired flag in the registry.
 // Requires the caller hold the registry lock.
 void ForEachFlagUnlocked(std::function<void(CommandLineFlag&)> visitor);

diff --git a/absl/functional/function_ref.h b/absl/functional/function_ref.h
index 370acc5..6e03ac2 100644
--- a/absl/functional/function_ref.h
+++ b/absl/functional/function_ref.h

@@ -90,7 +90,7 @@
   // Used to disable constructors for objects that are not compatible with the
   // signature of this FunctionRef.
   template <typename F,
-            typename FR = absl::base_internal::InvokeT<F, Args&&...>>
+            typename FR = absl::base_internal::invoke_result_t<F, Args&&...>>
   using EnableIfCompatible =
       typename std::enable_if<std::is_void<R>::value ||
                               std::is_convertible<FR, R>::value>::type;

diff --git a/absl/functional/internal/front_binder.h b/absl/functional/internal/front_binder.h
index a4d95da..45f52de 100644
--- a/absl/functional/internal/front_binder.h
+++ b/absl/functional/internal/front_binder.h

@@ -33,7 +33,7 @@
 // Invoke the method, expanding the tuple of bound arguments.
 template <class R, class Tuple, size_t... Idx, class... Args>
 R Apply(Tuple&& bound, absl::index_sequence<Idx...>, Args&&... free) {
-  return base_internal::Invoke(
+  return base_internal::invoke(
       absl::forward<Tuple>(bound).template get<Idx>()...,
       absl::forward<Args>(free)...);
 }
@@ -50,22 +50,22 @@
   constexpr explicit FrontBinder(absl::in_place_t, Ts&&... ts)
       : bound_args_(absl::forward<Ts>(ts)...) {}
 
-  template <class... FreeArgs,
-            class R = base_internal::InvokeT<F&, BoundArgs&..., FreeArgs&&...>>
+  template <class... FreeArgs, class R = base_internal::invoke_result_t<
+                                   F&, BoundArgs&..., FreeArgs&&...>>
   R operator()(FreeArgs&&... free_args) & {
     return functional_internal::Apply<R>(bound_args_, Idx(),
                                          absl::forward<FreeArgs>(free_args)...);
   }
 
   template <class... FreeArgs,
-            class R = base_internal::InvokeT<const F&, const BoundArgs&...,
-                                             FreeArgs&&...>>
+            class R = base_internal::invoke_result_t<
+                const F&, const BoundArgs&..., FreeArgs&&...>>
   R operator()(FreeArgs&&... free_args) const& {
     return functional_internal::Apply<R>(bound_args_, Idx(),
                                          absl::forward<FreeArgs>(free_args)...);
   }
 
-  template <class... FreeArgs, class R = base_internal::InvokeT<
+  template <class... FreeArgs, class R = base_internal::invoke_result_t<
                                    F&&, BoundArgs&&..., FreeArgs&&...>>
   R operator()(FreeArgs&&... free_args) && {
     // This overload is called when *this is an rvalue. If some of the bound
@@ -75,8 +75,8 @@
   }
 
   template <class... FreeArgs,
-            class R = base_internal::InvokeT<const F&&, const BoundArgs&&...,
-                                             FreeArgs&&...>>
+            class R = base_internal::invoke_result_t<
+                const F&&, const BoundArgs&&..., FreeArgs&&...>>
   R operator()(FreeArgs&&... free_args) const&& {
     // This overload is called when *this is an rvalue. If some of the bound
     // arguments are stored by value or rvalue reference, we move them.

diff --git a/absl/functional/internal/function_ref.h b/absl/functional/internal/function_ref.h
index d157505..b5bb8b4 100644
--- a/absl/functional/internal/function_ref.h
+++ b/absl/functional/internal/function_ref.h

@@ -71,14 +71,14 @@
 R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args) {
   auto o = static_cast<const Obj*>(ptr.obj);
   return static_cast<R>(
-      absl::base_internal::Invoke(*o, std::forward<Args>(args)...));
+      absl::base_internal::invoke(*o, std::forward<Args>(args)...));
 }
 
 template <typename Fun, typename R, typename... Args>
 R InvokeFunction(VoidPtr ptr, typename ForwardT<Args>::type... args) {
   auto f = reinterpret_cast<Fun>(ptr.fun);
   return static_cast<R>(
-      absl::base_internal::Invoke(f, std::forward<Args>(args)...));
+      absl::base_internal::invoke(f, std::forward<Args>(args)...));
 }
 
 template <typename Sig>

diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index ba87d2f..75689bb 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h

@@ -219,7 +219,7 @@
 // This metafunction is designed to be a drop-in replacement for the C++17
 // `std::conjunction` metafunction.
 template <typename... Ts>
-struct conjunction;
+struct conjunction : std::true_type {};
 
 template <typename T, typename... Ts>
 struct conjunction<T, Ts...>
@@ -228,9 +228,6 @@
 template <typename T>
 struct conjunction<T> : T {};
 
-template <>
-struct conjunction<> : std::true_type {};
-
 // disjunction
 //
 // Performs a compile-time logical OR operation on the passed types (which
@@ -241,7 +238,7 @@
 // This metafunction is designed to be a drop-in replacement for the C++17
 // `std::disjunction` metafunction.
 template <typename... Ts>
-struct disjunction;
+struct disjunction : std::false_type {};
 
 template <typename T, typename... Ts>
 struct disjunction<T, Ts...> :
@@ -250,9 +247,6 @@
 template <typename T>
 struct disjunction<T> : T {};
 
-template <>
-struct disjunction<> : std::false_type {};
-
 // negation
 //
 // Performs a compile-time logical NOT operation on the passed type (which

diff --git a/absl/numeric/BUILD.bazel b/absl/numeric/BUILD.bazel
index e09e52d..da3af4d 100644
--- a/absl/numeric/BUILD.bazel
+++ b/absl/numeric/BUILD.bazel

@@ -35,6 +35,7 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:bits",
         "//absl/base:config",
         "//absl/base:core_headers",
     ],

diff --git a/absl/numeric/CMakeLists.txt b/absl/numeric/CMakeLists.txt
index 242889f..1e12d80 100644
--- a/absl/numeric/CMakeLists.txt
+++ b/absl/numeric/CMakeLists.txt

@@ -26,6 +26,7 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::bits
     absl::config
     absl::core_headers
   PUBLIC

diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index b605a87..e21e5e9 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc

@@ -15,6 +15,7 @@
 #include "absl/numeric/int128.h"
 
 #include <stddef.h>
+
 #include <cassert>
 #include <iomanip>
 #include <ostream>  // NOLINT(readability/streams)
@@ -22,6 +23,9 @@
 #include <string>
 #include <type_traits>
 
+#include "absl/base/internal/bits.h"
+#include "absl/base/optimization.h"
+
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
@@ -31,44 +35,26 @@
 namespace {
 
 // Returns the 0-based position of the last set bit (i.e., most significant bit)
-// in the given uint64_t. The argument may not be 0.
+// in the given uint128. The argument is not 0.
 //
 // For example:
 //   Given: 5 (decimal) == 101 (binary)
 //   Returns: 2
-#define STEP(T, n, pos, sh)                   \
-  do {                                        \
-    if ((n) >= (static_cast<T>(1) << (sh))) { \
-      (n) = (n) >> (sh);                      \
-      (pos) |= (sh);                          \
-    }                                         \
-  } while (0)
-static inline int Fls64(uint64_t n) {
-  assert(n != 0);
-  int pos = 0;
-  STEP(uint64_t, n, pos, 0x20);
-  uint32_t n32 = static_cast<uint32_t>(n);
-  STEP(uint32_t, n32, pos, 0x10);
-  STEP(uint32_t, n32, pos, 0x08);
-  STEP(uint32_t, n32, pos, 0x04);
-  return pos + ((uint64_t{0x3333333322221100} >> (n32 << 2)) & 0x3);
-}
-#undef STEP
-
-// Like Fls64() above, but returns the 0-based position of the last set bit
-// (i.e., most significant bit) in the given uint128. The argument may not be 0.
-static inline int Fls128(uint128 n) {
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE int Fls128(uint128 n) {
   if (uint64_t hi = Uint128High64(n)) {
-    return Fls64(hi) + 64;
+    ABSL_INTERNAL_ASSUME(hi != 0);
+    return 127 - base_internal::CountLeadingZeros64(hi);
   }
-  return Fls64(Uint128Low64(n));
+  const uint64_t low = Uint128Low64(n);
+  ABSL_INTERNAL_ASSUME(low != 0);
+  return 63 - base_internal::CountLeadingZeros64(low);
 }
 
 // Long division/modulo for uint128 implemented using the shift-subtract
 // division algorithm adapted from:
 // https://stackoverflow.com/questions/5386377/division-without-using
-void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
-                uint128* remainder_ret) {
+inline void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
+                       uint128* remainder_ret) {
   assert(divisor != 0);
 
   if (divisor > dividend) {

diff --git a/absl/numeric/int128_benchmark.cc b/absl/numeric/int128_benchmark.cc
index a5502d9..eab1515 100644
--- a/absl/numeric/int128_benchmark.cc
+++ b/absl/numeric/int128_benchmark.cc

@@ -12,15 +12,15 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/numeric/int128.h"
-
 #include <algorithm>
 #include <cstdint>
+#include <limits>
 #include <random>
 #include <vector>
 
 #include "benchmark/benchmark.h"
 #include "absl/base/config.h"
+#include "absl/numeric/int128.h"
 
 namespace {
 
@@ -32,57 +32,85 @@
   return std::mt19937(seed);
 }
 
-std::vector<std::pair<absl::uint128, absl::uint128>>
-GetRandomClass128SampleUniformDivisor() {
-  std::vector<std::pair<absl::uint128, absl::uint128>> values;
+template <typename T,
+          typename H = typename std::conditional<
+              std::numeric_limits<T>::is_signed, int64_t, uint64_t>::type>
+std::vector<std::pair<T, T>> GetRandomClass128SampleUniformDivisor() {
+  std::vector<std::pair<T, T>> values;
   std::mt19937 random = MakeRandomEngine();
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+  std::uniform_int_distribution<H> uniform_h;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    absl::uint128 a =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    absl::uint128 b =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    values.emplace_back(std::max(a, b),
-                        std::max(absl::uint128(2), std::min(a, b)));
+    T a{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    T b{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    values.emplace_back(std::max(a, b), std::max(T(2), std::min(a, b)));
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideClass128UniformDivisor(benchmark::State& state) {
-  auto values = GetRandomClass128SampleUniformDivisor();
+  auto values = GetRandomClass128SampleUniformDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideClass128UniformDivisor);
+BENCHMARK_TEMPLATE(BM_DivideClass128UniformDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_DivideClass128UniformDivisor, absl::int128);
 
-std::vector<std::pair<absl::uint128, uint64_t>>
-GetRandomClass128SampleSmallDivisor() {
-  std::vector<std::pair<absl::uint128, uint64_t>> values;
+template <typename T>
+void BM_RemainderClass128UniformDivisor(benchmark::State& state) {
+  auto values = GetRandomClass128SampleUniformDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderClass128UniformDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_RemainderClass128UniformDivisor, absl::int128);
+
+template <typename T,
+          typename H = typename std::conditional<
+              std::numeric_limits<T>::is_signed, int64_t, uint64_t>::type>
+std::vector<std::pair<T, H>> GetRandomClass128SampleSmallDivisor() {
+  std::vector<std::pair<T, H>> values;
   std::mt19937 random = MakeRandomEngine();
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+  std::uniform_int_distribution<H> uniform_h;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    absl::uint128 a =
-        absl::MakeUint128(uniform_uint64(random), uniform_uint64(random));
-    uint64_t b = std::max(uint64_t{2}, uniform_uint64(random));
-    values.emplace_back(std::max(a, absl::uint128(b)), b);
+    T a{absl::MakeUint128(uniform_h(random), uniform_h(random))};
+    H b{std::max(H{2}, uniform_h(random))};
+    values.emplace_back(std::max(a, T(b)), b);
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideClass128SmallDivisor(benchmark::State& state) {
-  auto values = GetRandomClass128SampleSmallDivisor();
+  auto values = GetRandomClass128SampleSmallDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideClass128SmallDivisor);
+BENCHMARK_TEMPLATE(BM_DivideClass128SmallDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_DivideClass128SmallDivisor, absl::int128);
+
+template <typename T>
+void BM_RemainderClass128SmallDivisor(benchmark::State& state) {
+  auto values = GetRandomClass128SampleSmallDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderClass128SmallDivisor, absl::uint128);
+BENCHMARK_TEMPLATE(BM_RemainderClass128SmallDivisor, absl::int128);
 
 std::vector<std::pair<absl::uint128, absl::uint128>> GetRandomClass128Sample() {
   std::vector<std::pair<absl::uint128, absl::uint128>> values;
@@ -121,74 +149,107 @@
 
 // Some implementations of <random> do not support __int128 when it is
 // available, so we make our own uniform_int_distribution-like type.
+template <typename T,
+          typename H = typename std::conditional<
+              std::is_same<T, __int128>::value, int64_t, uint64_t>::type>
 class UniformIntDistribution128 {
  public:
   // NOLINTNEXTLINE: mimicking std::uniform_int_distribution API
-  unsigned __int128 operator()(std::mt19937& generator) {
-    return (static_cast<unsigned __int128>(dist64_(generator)) << 64) |
-           dist64_(generator);
+  T operator()(std::mt19937& generator) {
+    return (static_cast<T>(dist64_(generator)) << 64) | dist64_(generator);
   }
 
  private:
-  std::uniform_int_distribution<uint64_t> dist64_;
+  std::uniform_int_distribution<H> dist64_;
 };
 
-std::vector<std::pair<unsigned __int128, unsigned __int128>>
-GetRandomIntrinsic128SampleUniformDivisor() {
-  std::vector<std::pair<unsigned __int128, unsigned __int128>> values;
+template <typename T,
+          typename H = typename std::conditional<
+              std::is_same<T, __int128>::value, int64_t, uint64_t>::type>
+std::vector<std::pair<T, T>> GetRandomIntrinsic128SampleUniformDivisor() {
+  std::vector<std::pair<T, T>> values;
   std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
+  UniformIntDistribution128<T> uniform_128;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    unsigned __int128 a = uniform_uint128(random);
-    unsigned __int128 b = uniform_uint128(random);
-    values.emplace_back(
-        std::max(a, b),
-        std::max(static_cast<unsigned __int128>(2), std::min(a, b)));
+    T a = uniform_128(random);
+    T b = uniform_128(random);
+    values.emplace_back(std::max(a, b),
+                        std::max(static_cast<T>(2), std::min(a, b)));
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideIntrinsic128UniformDivisor(benchmark::State& state) {
-  auto values = GetRandomIntrinsic128SampleUniformDivisor();
+  auto values = GetRandomIntrinsic128SampleUniformDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideIntrinsic128UniformDivisor);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128UniformDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128UniformDivisor, __int128);
 
-std::vector<std::pair<unsigned __int128, uint64_t>>
-GetRandomIntrinsic128SampleSmallDivisor() {
-  std::vector<std::pair<unsigned __int128, uint64_t>> values;
+template <typename T>
+void BM_RemainderIntrinsic128UniformDivisor(benchmark::State& state) {
+  auto values = GetRandomIntrinsic128SampleUniformDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128UniformDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128UniformDivisor, __int128);
+
+template <typename T,
+          typename H = typename std::conditional<
+              std::is_same<T, __int128>::value, int64_t, uint64_t>::type>
+std::vector<std::pair<T, H>> GetRandomIntrinsic128SampleSmallDivisor() {
+  std::vector<std::pair<T, H>> values;
   std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
-  std::uniform_int_distribution<uint64_t> uniform_uint64;
+  UniformIntDistribution128<T> uniform_int128;
+  std::uniform_int_distribution<H> uniform_int64;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
-    unsigned __int128 a = uniform_uint128(random);
-    uint64_t b = std::max(uint64_t{2}, uniform_uint64(random));
-    values.emplace_back(std::max(a, static_cast<unsigned __int128>(b)), b);
+    T a = uniform_int128(random);
+    H b = std::max(H{2}, uniform_int64(random));
+    values.emplace_back(std::max(a, static_cast<T>(b)), b);
   }
   return values;
 }
 
+template <typename T>
 void BM_DivideIntrinsic128SmallDivisor(benchmark::State& state) {
-  auto values = GetRandomIntrinsic128SampleSmallDivisor();
+  auto values = GetRandomIntrinsic128SampleSmallDivisor<T>();
   while (state.KeepRunningBatch(values.size())) {
     for (const auto& pair : values) {
       benchmark::DoNotOptimize(pair.first / pair.second);
     }
   }
 }
-BENCHMARK(BM_DivideIntrinsic128SmallDivisor);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128SmallDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_DivideIntrinsic128SmallDivisor, __int128);
+
+template <typename T>
+void BM_RemainderIntrinsic128SmallDivisor(benchmark::State& state) {
+  auto values = GetRandomIntrinsic128SampleSmallDivisor<T>();
+  while (state.KeepRunningBatch(values.size())) {
+    for (const auto& pair : values) {
+      benchmark::DoNotOptimize(pair.first % pair.second);
+    }
+  }
+}
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128SmallDivisor, unsigned __int128);
+BENCHMARK_TEMPLATE(BM_RemainderIntrinsic128SmallDivisor, __int128);
 
 std::vector<std::pair<unsigned __int128, unsigned __int128>>
       GetRandomIntrinsic128Sample() {
   std::vector<std::pair<unsigned __int128, unsigned __int128>> values;
   std::mt19937 random = MakeRandomEngine();
-  UniformIntDistribution128 uniform_uint128;
+  UniformIntDistribution128<unsigned __int128> uniform_uint128;
   values.reserve(kSampleSize);
   for (size_t i = 0; i < kSampleSize; ++i) {
     values.emplace_back(uniform_uint128(random), uniform_uint128(random));

diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 8875b30..ade9ea1 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt

@@ -69,6 +69,21 @@
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
+    random_internal_mock_helpers
+  HDRS
+    "internal/mock_helpers.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::fast_type_id
+    absl::optional
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
     random_internal_mock_overload_set
   HDRS
     "internal/mock_overload_set.h"
@@ -78,7 +93,7 @@
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_mocking_bit_gen
-    absl::fast_type_id
+    absl::random_internal_mock_helpers
   TESTONLY
 )
 

diff --git a/absl/random/bit_gen_ref.h b/absl/random/bit_gen_ref.h
index 00e3624..9555460 100644
--- a/absl/random/bit_gen_ref.h
+++ b/absl/random/bit_gen_ref.h

@@ -53,6 +53,7 @@
 
 template <typename>
 struct DistributionCaller;
+class MockHelpers;
 
 }  // namespace random_internal
 
@@ -171,6 +172,7 @@
 
   template <typename>
   friend struct ::absl::random_internal::DistributionCaller;  // for InvokeMock
+  friend class ::absl::random_internal::MockHelpers;          // for InvokeMock
 };
 
 ABSL_NAMESPACE_END

diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index 3ab1177..d81477f 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel

@@ -485,11 +485,20 @@
 )
 
 cc_library(
+    name = "mock_helpers",
+    hdrs = ["mock_helpers.h"],
+    deps = [
+        "//absl/base:fast_type_id",
+        "//absl/types:optional",
+    ],
+)
+
+cc_library(
     name = "mock_overload_set",
     testonly = 1,
     hdrs = ["mock_overload_set.h"],
     deps = [
-        "//absl/base:fast_type_id",
+        ":mock_helpers",
         "//absl/random:mocking_bit_gen",
         "@com_google_googletest//:gtest",
     ],

diff --git a/absl/random/internal/distribution_caller.h b/absl/random/internal/distribution_caller.h
index 87a7684..fc81b78 100644
--- a/absl/random/internal/distribution_caller.h
+++ b/absl/random/internal/distribution_caller.h

@@ -52,23 +52,25 @@
 
   // Default implementation of distribution caller.
   template <typename DistrT, typename... Args>
-  static inline typename DistrT::result_type Impl(std::false_type, URBG* urbg,
-                                                  Args&&... args) {
+  static typename DistrT::result_type Impl(std::false_type, URBG* urbg,
+                                           Args&&... args) {
     DistrT dist(std::forward<Args>(args)...);
     return dist(*urbg);
   }
 
   // Mock implementation of distribution caller.
+  // The underlying KeyT must match the KeyT constructed by MockOverloadSet.
   template <typename DistrT, typename... Args>
-  static inline typename DistrT::result_type Impl(std::true_type, URBG* urbg,
-                                                  Args&&... args) {
+  static typename DistrT::result_type Impl(std::true_type, URBG* urbg,
+                                           Args&&... args) {
     using ResultT = typename DistrT::result_type;
     using ArgTupleT = std::tuple<absl::decay_t<Args>...>;
+    using KeyT = ResultT(DistrT, ArgTupleT);
+
     ArgTupleT arg_tuple(std::forward<Args>(args)...);
     ResultT result;
-    if (!urbg->InvokeMock(
-            ::absl::base_internal::FastTypeId<ResultT(DistrT, ArgTupleT)>(),
-            &arg_tuple, &result)) {
+    if (!urbg->InvokeMock(::absl::base_internal::FastTypeId<KeyT>(), &arg_tuple,
+                          &result)) {
       auto dist = absl::make_from_tuple<DistrT>(arg_tuple);
       result = dist(*urbg);
     }
@@ -77,7 +79,7 @@
 
   // Default implementation of distribution caller.
   template <typename DistrT, typename... Args>
-  static inline typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
+  static typename DistrT::result_type Call(URBG* urbg, Args&&... args) {
     return Impl<DistrT, Args...>(HasInvokeMock{}, urbg,
                                  std::forward<Args>(args)...);
   }

diff --git a/absl/random/internal/mock_helpers.h b/absl/random/internal/mock_helpers.h
new file mode 100644
index 0000000..9af27ab
--- /dev/null
+++ b/absl/random/internal/mock_helpers.h

@@ -0,0 +1,127 @@
+//
+// 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_RANDOM_INTERNAL_MOCK_HELPERS_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_
+
+#include <tuple>
+#include <type_traits>
+
+#include "absl/base/internal/fast_type_id.h"
+#include "absl/types/optional.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// MockHelpers works in conjunction with MockOverloadSet, MockingBitGen, and
+// BitGenRef to enable the mocking capability for absl distribution functions.
+//
+// MockingBitGen registers mocks based on the typeid of a mock signature, KeyT,
+// which is used to generate a unique id.
+//
+// KeyT is a signature of the form:
+//   result_type(discriminator_type, std::tuple<args...>)
+// The mocked function signature will be composed from KeyT as:
+//   result_type(args...)
+//
+class MockHelpers {
+  using IdType = ::absl::base_internal::FastTypeIdType;
+
+  // Given a key signature type used to index the mock, extract the components.
+  // KeyT is expected to have the form:
+  //   result_type(discriminator_type, arg_tuple_type)
+  template <typename KeyT>
+  struct KeySignature;
+
+  template <typename ResultT, typename DiscriminatorT, typename ArgTupleT>
+  struct KeySignature<ResultT(DiscriminatorT, ArgTupleT)> {
+    using result_type = ResultT;
+    using discriminator_type = DiscriminatorT;
+    using arg_tuple_type = ArgTupleT;
+  };
+
+  // Detector for InvokeMock.
+  template <class T>
+  using invoke_mock_t = decltype(std::declval<T*>()->InvokeMock(
+      std::declval<IdType>(), std::declval<void*>(), std::declval<void*>()));
+
+  // Empty implementation of InvokeMock.
+  template <typename KeyT, typename ReturnT, typename ArgTupleT, typename URBG,
+            typename... Args>
+  static absl::optional<ReturnT> InvokeMockImpl(char, URBG*, Args&&...) {
+    return absl::nullopt;
+  }
+
+  // Non-empty implementation of InvokeMock.
+  template <typename KeyT, typename ReturnT, typename ArgTupleT, typename URBG,
+            typename = invoke_mock_t<URBG>, typename... Args>
+  static absl::optional<ReturnT> InvokeMockImpl(int, URBG* urbg,
+                                                Args&&... args) {
+    ArgTupleT arg_tuple(std::forward<Args>(args)...);
+    ReturnT result;
+    if (urbg->InvokeMock(::absl::base_internal::FastTypeId<KeyT>(), &arg_tuple,
+                         &result)) {
+      return result;
+    }
+    return absl::nullopt;
+  }
+
+ public:
+  // Invoke a mock for the KeyT (may or may not be a signature).
+  //
+  // KeyT is used to generate a typeid-based lookup key for the mock.
+  // KeyT is a signature of the form:
+  //   result_type(discriminator_type, std::tuple<args...>)
+  // The mocked function signature will be composed from KeyT as:
+  //   result_type(args...)
+  //
+  // An instance of arg_tuple_type must be constructable from Args..., since
+  // the underlying mechanism requires a pointer to an argument tuple.
+  template <typename KeyT, typename URBG, typename... Args>
+  static auto MaybeInvokeMock(URBG* urbg, Args&&... args)
+      -> absl::optional<typename KeySignature<KeyT>::result_type> {
+    // Use function overloading to dispatch to the implemenation since
+    // more modern patterns (e.g. require + constexpr) are not supported in all
+    // compiler configurations.
+    return InvokeMockImpl<KeyT, typename KeySignature<KeyT>::result_type,
+                          typename KeySignature<KeyT>::arg_tuple_type, URBG>(
+        0, urbg, std::forward<Args>(args)...);
+  }
+
+  // Acquire a mock for the KeyT (may or may not be a signature).
+  //
+  // KeyT is used to generate a typeid-based lookup for the mock.
+  // KeyT is a signature of the form:
+  //   result_type(discriminator_type, std::tuple<args...>)
+  // The mocked function signature will be composed from KeyT as:
+  //   result_type(args...)
+  template <typename KeyT, typename MockURBG>
+  static auto MockFor(MockURBG& m) -> decltype(
+      std::declval<MockURBG>()
+          .template RegisterMock<typename KeySignature<KeyT>::result_type,
+                                 typename KeySignature<KeyT>::arg_tuple_type>(
+              std::declval<IdType>())) {
+    return m.template RegisterMock<typename KeySignature<KeyT>::result_type,
+                                   typename KeySignature<KeyT>::arg_tuple_type>(
+        ::absl::base_internal::FastTypeId<KeyT>());
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_HELPERS_H_

diff --git a/absl/random/internal/mock_overload_set.h b/absl/random/internal/mock_overload_set.h
index 60561b5..dccc6ce 100644
--- a/absl/random/internal/mock_overload_set.h
+++ b/absl/random/internal/mock_overload_set.h

@@ -20,7 +20,7 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/internal/fast_type_id.h"
+#include "absl/random/internal/mock_helpers.h"
 #include "absl/random/mocking_bit_gen.h"
 
 namespace absl {
@@ -37,22 +37,19 @@
 // `mock_single_overload.gmock_Call(...)`. Because expectations are stored on
 // the MockingBitGen (an argument passed inside `Call(...)`), this forwards to
 // arguments to MockingBitGen::Register.
+//
+// The underlying KeyT must match the KeyT constructed by DistributionCaller.
 template <typename DistrT, typename Ret, typename... Args>
 struct MockSingleOverload<DistrT, Ret(MockingBitGen&, Args...)> {
   static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
                 "Overload signature must have return type matching the "
                 "distribution result_type.");
-  using ArgTupleT = std::tuple<Args...>;
+  using KeyT = Ret(DistrT, std::tuple<Args...>);
   auto gmock_Call(
       absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
       const ::testing::Matcher<Args>&... matchers)
-      -> decltype(gen.RegisterMock<Ret, ArgTupleT>(
-                         std::declval<::absl::base_internal::FastTypeIdType>())
-                      .gmock_Call(matchers...)) {
-    return gen
-        .RegisterMock<Ret, ArgTupleT>(
-            ::absl::base_internal::FastTypeId<Ret(DistrT, ArgTupleT)>())
-        .gmock_Call(matchers...);
+      -> decltype(MockHelpers::MockFor<KeyT>(gen).gmock_Call(matchers...)) {
+    return MockHelpers::MockFor<KeyT>(gen).gmock_Call(matchers...);
   }
 };
 
@@ -61,18 +58,14 @@
   static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
                 "Overload signature must have return type matching the "
                 "distribution result_type.");
-  using ArgTupleT = std::tuple<Arg, Args...>;
+  using KeyT = Ret(DistrT, std::tuple<Arg, Args...>);
   auto gmock_Call(
       const ::testing::Matcher<Arg>& matcher,
       absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
       const ::testing::Matcher<Args>&... matchers)
-      -> decltype(gen.RegisterMock<Ret, ArgTupleT>(
-                         std::declval<::absl::base_internal::FastTypeIdType>())
-                      .gmock_Call(matcher, matchers...)) {
-    return gen
-        .RegisterMock<Ret, ArgTupleT>(
-            ::absl::base_internal::FastTypeId<Ret(DistrT, ArgTupleT)>())
-        .gmock_Call(matcher, matchers...);
+      -> decltype(MockHelpers::MockFor<KeyT>(gen).gmock_Call(matcher,
+                                                             matchers...)) {
+    return MockHelpers::MockFor<KeyT>(gen).gmock_Call(matcher, matchers...);
   }
 };
 

diff --git a/absl/random/mocking_bit_gen.h b/absl/random/mocking_bit_gen.h
index c3c7d44..6d2f2c8 100644
--- a/absl/random/mocking_bit_gen.h
+++ b/absl/random/mocking_bit_gen.h

@@ -53,10 +53,9 @@
 ABSL_NAMESPACE_BEGIN
 
 namespace random_internal {
-template <typename, typename>
-struct MockSingleOverload;
 template <typename>
 struct DistributionCaller;
+class MockHelpers;
 
 }  // namespace random_internal
 class BitGenRef;
@@ -216,11 +215,11 @@
   std::vector<std::function<void()>> deleters_;
   absl::BitGen gen_;
 
-  template <typename, typename>
-  friend struct ::absl::random_internal::MockSingleOverload;  // for Register
   template <typename>
   friend struct ::absl::random_internal::DistributionCaller;  // for InvokeMock
   friend class ::absl::BitGenRef;                             // for InvokeMock
+  friend class ::absl::random_internal::MockHelpers;  // for RegisterMock,
+                                                      // InvokeMock
 };
 
 ABSL_NAMESPACE_END

diff --git a/absl/random/seed_sequences_test.cc b/absl/random/seed_sequences_test.cc
index 2cc8b0e..fe1100b 100644
--- a/absl/random/seed_sequences_test.cc
+++ b/absl/random/seed_sequences_test.cc

@@ -96,7 +96,6 @@
 void TestReproducibleVariateSequencesForNonsecureURBG() {
   const size_t kNumVariates = 1000;
 
-  // Master RNG instance.
   URBG rng;
   // Reused for both RNG instances.
   auto reusable_seed = absl::CreateSeedSeqFrom(&rng);

diff --git a/absl/status/status.cc b/absl/status/status.cc
index 6d57a6b..0a65573 100644
--- a/absl/status/status.cc
+++ b/absl/status/status.cc

@@ -27,8 +27,6 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-// The implementation was intentionally kept same as util::error::Code_Name()
-// to ease the migration.
 std::string StatusCodeToString(StatusCode code) {
   switch (code) {
     case StatusCode::kOk:

diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index dc98745..8580d80 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h

@@ -857,16 +857,16 @@
 struct Rank1 {};
 struct Rank0 : Rank1 {};
 
-template <typename Releaser, typename = ::absl::base_internal::InvokeT<
+template <typename Releaser, typename = ::absl::base_internal::invoke_result_t<
                                  Releaser, absl::string_view>>
 void InvokeReleaser(Rank0, Releaser&& releaser, absl::string_view data) {
-  ::absl::base_internal::Invoke(std::forward<Releaser>(releaser), data);
+  ::absl::base_internal::invoke(std::forward<Releaser>(releaser), data);
 }
 
 template <typename Releaser,
-          typename = ::absl::base_internal::InvokeT<Releaser>>
+          typename = ::absl::base_internal::invoke_result_t<Releaser>>
 void InvokeReleaser(Rank1, Releaser&& releaser, absl::string_view) {
-  ::absl::base_internal::Invoke(std::forward<Releaser>(releaser));
+  ::absl::base_internal::invoke(std::forward<Releaser>(releaser));
 }
 
 // Creates a new `CordRep` that owns `data` and `releaser` and returns a pointer

diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index d441e87..3dbc152 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h

@@ -25,10 +25,12 @@
 class FormatCountCapture;
 class FormatSink;
 
-namespace str_format_internal {
-
+template <absl::FormatConversionCharSet C>
+struct FormatConvertResult;
 class FormatConversionSpec;
 
+namespace str_format_internal {
+
 template <typename T, typename = void>
 struct HasUserDefinedConvert : std::false_type {};
 
@@ -39,6 +41,22 @@
                                     std::declval<FormatSink*>()))>>
     : std::true_type {};
 
+void AbslFormatConvert();  // Stops the lexical name lookup
+template <typename T>
+auto FormatConvertImpl(const T& v, FormatConversionSpecImpl conv,
+                       FormatSinkImpl* sink)
+    -> decltype(AbslFormatConvert(v,
+                                  std::declval<const FormatConversionSpec&>(),
+                                  std::declval<FormatSink*>())) {
+  using FormatConversionSpecT =
+      absl::enable_if_t<sizeof(const T& (*)()) != 0, FormatConversionSpec>;
+  using FormatSinkT =
+      absl::enable_if_t<sizeof(const T& (*)()) != 0, FormatSink>;
+  auto fcs = conv.Wrap<FormatConversionSpecT>();
+  auto fs = sink->Wrap<FormatSinkT>();
+  return AbslFormatConvert(v, fcs, &fs);
+}
+
 template <typename T>
 class StreamedWrapper;
 
@@ -46,6 +64,13 @@
 // then convert it, appending to `sink` and return `true`.
 // Otherwise fail and return `false`.
 
+// AbslFormatConvert(v, conv, sink) is intended to be found by ADL on 'v'
+// as an extension mechanism. These FormatConvertImpl functions are the default
+// implementations.
+// The ADL search is augmented via the 'Sink*' parameter, which also
+// serves as a disambiguator to reject possible unintended 'AbslFormatConvert'
+// functions in the namespaces associated with 'v'.
+
 // Raw pointers.
 struct VoidPtr {
   VoidPtr() = default;
@@ -62,6 +87,11 @@
 };
 
 template <FormatConversionCharSet C>
+constexpr FormatConversionCharSet ExtractCharSet(FormatConvertResult<C>) {
+  return C;
+}
+
+template <FormatConversionCharSet C>
 constexpr FormatConversionCharSet ExtractCharSet(ArgConvertResult<C>) {
   return C;
 }

diff --git a/absl/strings/internal/str_format/arg_test.cc b/absl/strings/internal/str_format/arg_test.cc
index bf3d7e8..f53fd6b 100644
--- a/absl/strings/internal/str_format/arg_test.cc
+++ b/absl/strings/internal/str_format/arg_test.cc

@@ -23,8 +23,17 @@
   enum Color { kRed, kGreen, kBlue };
 
   static const char *hi() { return "hi"; }
+
+  struct X {};
+
+  X x_;
 };
 
+inline FormatConvertResult<FormatConversionCharSet{}> AbslFormatConvert(
+    const FormatArgImplTest::X &, const FormatConversionSpec &, FormatSink *) {
+  return {false};
+}
+
 TEST_F(FormatArgImplTest, ToInt) {
   int out = 0;
   EXPECT_TRUE(FormatArgImplFriend::ToInt(FormatArgImpl(1), &out));
@@ -59,6 +68,7 @@
       FormatArgImpl(static_cast<int *>(nullptr)), &out));
   EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl(hi()), &out));
   EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl("hi"), &out));
+  EXPECT_FALSE(FormatArgImplFriend::ToInt(FormatArgImpl(x_), &out));
   EXPECT_TRUE(FormatArgImplFriend::ToInt(FormatArgImpl(kBlue), &out));
   EXPECT_EQ(2, out);
 }

diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index 94f2b9c..bb0d96c 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc

@@ -33,6 +33,29 @@
   return s;
 }
 
+#define ABSL_INTERNAL_X_VAL(id) \
+  constexpr absl::FormatConversionChar FormatConversionCharInternal::id;
+ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
+#undef ABSL_INTERNAL_X_VAL
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr absl::FormatConversionChar FormatConversionCharInternal::kNone;
+
+#define ABSL_INTERNAL_CHAR_SET_CASE(c) \
+  constexpr FormatConversionCharSet FormatConversionCharSetInternal::c;
+ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
+#undef ABSL_INTERNAL_CHAR_SET_CASE
+
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr FormatConversionCharSet FormatConversionCharSetInternal::kStar;
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr FormatConversionCharSet FormatConversionCharSetInternal::kIntegral;
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr FormatConversionCharSet FormatConversionCharSetInternal::kFloating;
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr FormatConversionCharSet FormatConversionCharSetInternal::kNumeric;
+// NOLINTNEXTLINE(readability-redundant-declaration)
+constexpr FormatConversionCharSet FormatConversionCharSetInternal::kPointer;
+
 bool FormatSinkImpl::PutPaddedString(string_view value, int width,
                                      int precision, bool left) {
   size_t space_remaining = 0;

diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index 6c60c6c..a9b9e13 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h

@@ -31,11 +31,11 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
-namespace str_format_internal {
-
 enum class FormatConversionChar : uint8_t;
 enum class FormatConversionCharSet : uint64_t;
 
+namespace str_format_internal {
+
 class FormatRawSinkImpl {
  public:
   // Implicitly convert from any type that provides the hook function as
@@ -361,14 +361,12 @@
   static constexpr FormatConversionCharSet kStar =
       FormatConversionCharToConvValue('*');
 
-  // Some predefined values (TODO(matthewbr), delete any that are unused).
   static constexpr FormatConversionCharSet kIntegral =
       FormatConversionCharSetUnion(d, i, u, o, x, X);
   static constexpr FormatConversionCharSet kFloating =
       FormatConversionCharSetUnion(a, e, f, g, A, E, F, G);
   static constexpr FormatConversionCharSet kNumeric =
       FormatConversionCharSetUnion(kIntegral, kFloating);
-  static constexpr FormatConversionCharSet kString = s;
   static constexpr FormatConversionCharSet kPointer = p;
 };
 

diff --git a/absl/strings/internal/str_format/extension_test.cc b/absl/strings/internal/str_format/extension_test.cc
index 0a023f9..1c93fdb 100644
--- a/absl/strings/internal/str_format/extension_test.cc
+++ b/absl/strings/internal/str_format/extension_test.cc

@@ -80,4 +80,19 @@
     EXPECT_EQ(actual, expected);
   }
 }
+
+TEST(FormatExtensionTest, VerifyEnumEquality) {
+#define X_VAL(id)                           \
+  EXPECT_EQ(absl::FormatConversionChar::id, \
+            absl::str_format_internal::FormatConversionCharInternal::id);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, );
+#undef X_VAL
+
+#define X_VAL(id)                              \
+  EXPECT_EQ(absl::FormatConversionCharSet::id, \
+            absl::str_format_internal::FormatConversionCharSetInternal::id);
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, );
+#undef X_VAL
+}
+
 }  // namespace

diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index 36bd84a..0146510 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h

@@ -63,6 +63,9 @@
 //     loosely typed. `FormatUntyped()` is not a template and does not perform
 //     any compile-time checking of the format string; instead, it returns a
 //     boolean from a runtime check.
+//
+// In addition, the `str_format` library provides extension points for
+// augmenting formatting to new types.  See "StrFormat Extensions" below.
 
 #ifndef ABSL_STRINGS_STR_FORMAT_H_
 #define ABSL_STRINGS_STR_FORMAT_H_
@@ -278,9 +281,36 @@
 //   } else {
 //     ... error case ...
 //   }
+
+#if defined(__cpp_nontype_template_parameter_auto)
+// If C++17 is available, an 'extended' format is also allowed that can specify
+// multiple conversion characters per format argument, using a combination of
+// `absl::FormatConversionCharSet` enum values (logically a set union)
+//  via the `|` operator. (Single character-based arguments are still accepted,
+// but cannot be combined). Some common conversions also have predefined enum
+// values, such as `absl::FormatConversionCharSet::kIntegral`.
+//
+// Example:
+//   // Extended format supports multiple conversion characters per argument,
+//   // specified via a combination of `FormatConversionCharSet` enums.
+//   using MyFormat = absl::ParsedFormat<absl::FormatConversionCharSet::d |
+//                                       absl::FormatConversionCharSet::x>;
+//   MyFormat GetFormat(bool use_hex) {
+//     if (use_hex) return MyFormat("foo %x bar");
+//     return MyFormat("foo %d bar");
+//   }
+//   // `format` can be used with any value that supports 'd' and 'x',
+//   // like `int`.
+//   auto format = GetFormat(use_hex);
+//   value = StringF(format, i);
+template <auto... Conv>
+using ParsedFormat = absl::str_format_internal::ExtendedParsedFormat<
+    absl::str_format_internal::ToFormatConversionCharSet(Conv)...>;
+#else
 template <char... Conv>
 using ParsedFormat = str_format_internal::ExtendedParsedFormat<
     absl::str_format_internal::ToFormatConversionCharSet(Conv)...>;
+#endif  // defined(__cpp_nontype_template_parameter_auto)
 
 // StrFormat()
 //
@@ -537,6 +567,246 @@
       str_format_internal::UntypedFormatSpecImpl::Extract(format), args);
 }
 
+//------------------------------------------------------------------------------
+// StrFormat Extensions
+//------------------------------------------------------------------------------
+//
+// AbslFormatConvert()
+//
+// The StrFormat library provides a customization API for formatting
+// user-defined types using absl::StrFormat(). The API relies on detecting an
+// overload in the user-defined type's namespace of a free (non-member)
+// `AbslFormatConvert()` function, usually as a friend definition with the
+// following signature:
+//
+// absl::FormatConvertResult<...> AbslFormatConvert(
+//     const X& value,
+//     const absl::FormatConversionSpec& spec,
+//     absl::FormatSink *sink);
+//
+// An `AbslFormatConvert()` overload for a type should only be declared in the
+// same file and namespace as said type.
+//
+// The abstractions within this definition include:
+//
+// * An `absl::FormatConversionSpec` to specify the fields to pull from a
+//   user-defined type's format string
+// * An `absl::FormatSink` to hold the converted string data during the
+//   conversion process.
+// * An `absl::FormatConvertResult` to hold the status of the returned
+//   formatting operation
+//
+// The return type encodes all the conversion characters that your
+// AbslFormatConvert() routine accepts.  The return value should be {true}.
+// A return value of {false} will result in `StrFormat()` returning
+// an empty string.  This result will be propagated to the result of
+// `FormatUntyped`.
+//
+// Example:
+//
+// struct Point {
+//   // To add formatting support to `Point`, we simply need to add a free
+//   // (non-member) function `AbslFormatConvert()`.  This method interprets
+//   // `spec` to print in the request format. The allowed conversion characters
+//   // can be restricted via the type of the result, in this example
+//   // string and integral formatting are allowed (but not, for instance
+//   // floating point characters like "%f").  You can add such a free function
+//   // using a friend declaration within the body of the class:
+//   friend absl::FormatConvertResult<absl::FormatConversionCharSet::kString |
+//                                    absl::FormatConversionCharSet::kIntegral>
+//   AbslFormatConvert(const Point& p, const absl::FormatConversionSpec& spec,
+//                     absl::FormatSink* s) {
+//     if (spec.conversion_char() == absl::FormatConversionChar::s) {
+//       s->Append(absl::StrCat("x=", p.x, " y=", p.y));
+//     } else {
+//       s->Append(absl::StrCat(p.x, ",", p.y));
+//     }
+//     return {true};
+//   }
+//
+//   int x;
+//   int y;
+// };
+
+// clang-format off
+
+// FormatConversionChar
+//
+// Specifies the formatting character provided in the format string
+// passed to `StrFormat()`.
+enum class FormatConversionChar : uint8_t {
+  c, s,                    // text
+  d, i, o, u, x, X,        // int
+  f, F, e, E, g, G, a, A,  // float
+  n, p                     // misc
+};
+// clang-format on
+
+// FormatConversionSpec
+//
+// Specifies modifications to the conversion of the format string, through use
+// of one or more format flags in the source format string.
+class FormatConversionSpec {
+ public:
+  // FormatConversionSpec::is_basic()
+  //
+  // Indicates that width and precision are not specified, and no additional
+  // flags are set for this conversion character in the format string.
+  bool is_basic() const { return impl_.is_basic(); }
+
+  // FormatConversionSpec::has_left_flag()
+  //
+  // Indicates whether the result should be left justified for this conversion
+  // character in the format string. This flag is set through use of a '-'
+  // character in the format string. E.g. "%-s"
+  bool has_left_flag() const { return impl_.has_left_flag(); }
+
+  // FormatConversionSpec::has_show_pos_flag()
+  //
+  // Indicates whether a sign column is prepended to the result for this
+  // conversion character in the format string, even if the result is positive.
+  // This flag is set through use of a '+' character in the format string.
+  // E.g. "%+d"
+  bool has_show_pos_flag() const { return impl_.has_show_pos_flag(); }
+
+  // FormatConversionSpec::has_sign_col_flag()
+  //
+  // Indicates whether a mandatory sign column is added to the result for this
+  // conversion character. This flag is set through use of a space character
+  // (' ') in the format string. E.g. "% i"
+  bool has_sign_col_flag() const { return impl_.has_sign_col_flag(); }
+
+  // FormatConversionSpec::has_alt_flag()
+  //
+  // Indicates whether an "alternate" format is applied to the result for this
+  // conversion character. Alternative forms depend on the type of conversion
+  // character, and unallowed alternatives are undefined. This flag is set
+  // through use of a '#' character in the format string. E.g. "%#h"
+  bool has_alt_flag() const { return impl_.has_alt_flag(); }
+
+  // FormatConversionSpec::has_zero_flag()
+  //
+  // Indicates whether zeroes should be prepended to the result for this
+  // conversion character instead of spaces. This flag is set through use of the
+  // '0' character in the format string. E.g. "%0f"
+  bool has_zero_flag() const { return impl_.has_zero_flag(); }
+
+  // FormatConversionSpec::conversion_char()
+  //
+  // Returns the underlying conversion character.
+  FormatConversionChar conversion_char() const {
+    return impl_.conversion_char();
+  }
+
+  // FormatConversionSpec::width()
+  //
+  // Returns the specified width (indicated through use of a non-zero integer
+  // value or '*' character) of the conversion character. If width is
+  // unspecified, it returns a negative value.
+  int width() const { return impl_.width(); }
+
+  // FormatConversionSpec::precision()
+  //
+  // Returns the specified precision (through use of the '.' character followed
+  // by a non-zero integer value or '*' character) of the conversion character.
+  // If precision is unspecified, it returns a negative value.
+  int precision() const { return impl_.precision(); }
+
+ private:
+  explicit FormatConversionSpec(
+      str_format_internal::FormatConversionSpecImpl impl)
+      : impl_(impl) {}
+
+  friend str_format_internal::FormatConversionSpecImpl;
+
+  absl::str_format_internal::FormatConversionSpecImpl impl_;
+};
+
+// Type safe OR operator for FormatConversionCharSet to allow accepting multiple
+// conversion chars in custom format converters.
+constexpr FormatConversionCharSet operator|(FormatConversionCharSet a,
+                                            FormatConversionCharSet b) {
+  return static_cast<FormatConversionCharSet>(static_cast<uint64_t>(a) |
+                                              static_cast<uint64_t>(b));
+}
+
+// FormatConversionCharSet
+//
+// Specifies the _accepted_ conversion types as a template parameter to
+// FormatConvertResult for custom implementations of `AbslFormatConvert`.
+// Note the helper predefined alias definitions (kIntegral, etc.) below.
+enum class FormatConversionCharSet : uint64_t {
+  // text
+  c = str_format_internal::FormatConversionCharToConvInt('c'),
+  s = str_format_internal::FormatConversionCharToConvInt('s'),
+  // integer
+  d = str_format_internal::FormatConversionCharToConvInt('d'),
+  i = str_format_internal::FormatConversionCharToConvInt('i'),
+  o = str_format_internal::FormatConversionCharToConvInt('o'),
+  u = str_format_internal::FormatConversionCharToConvInt('u'),
+  x = str_format_internal::FormatConversionCharToConvInt('x'),
+  X = str_format_internal::FormatConversionCharToConvInt('X'),
+  // Float
+  f = str_format_internal::FormatConversionCharToConvInt('f'),
+  F = str_format_internal::FormatConversionCharToConvInt('F'),
+  e = str_format_internal::FormatConversionCharToConvInt('e'),
+  E = str_format_internal::FormatConversionCharToConvInt('E'),
+  g = str_format_internal::FormatConversionCharToConvInt('g'),
+  G = str_format_internal::FormatConversionCharToConvInt('G'),
+  a = str_format_internal::FormatConversionCharToConvInt('a'),
+  A = str_format_internal::FormatConversionCharToConvInt('A'),
+  // misc
+  n = str_format_internal::FormatConversionCharToConvInt('n'),
+  p = str_format_internal::FormatConversionCharToConvInt('p'),
+
+  // Used for width/precision '*' specification.
+  kStar = static_cast<uint64_t>(
+      absl::str_format_internal::FormatConversionCharSetInternal::kStar),
+  // Some predefined values:
+  kIntegral = d | i | u | o | x | X,
+  kFloating = a | e | f | g | A | E | F | G,
+  kNumeric = kIntegral | kFloating,
+  kString = s,
+  kPointer = p,
+};
+
+// FormatSink
+//
+// An abstraction to which conversions write their string data.
+//
+class FormatSink {
+ public:
+  // Appends `count` copies of `ch`.
+  void Append(size_t count, char ch) { sink_->Append(count, ch); }
+
+  void Append(string_view v) { sink_->Append(v); }
+
+  // Appends the first `precision` bytes of `v`. If this is less than
+  // `width`, spaces will be appended first (if `left` is false), or
+  // after (if `left` is true) to ensure the total amount appended is
+  // at least `width`.
+  bool PutPaddedString(string_view v, int width, int precision, bool left) {
+    return sink_->PutPaddedString(v, width, precision, left);
+  }
+
+ private:
+  friend str_format_internal::FormatSinkImpl;
+  explicit FormatSink(str_format_internal::FormatSinkImpl* s) : sink_(s) {}
+  str_format_internal::FormatSinkImpl* sink_;
+};
+
+// FormatConvertResult
+//
+// Indicates whether a call to AbslFormatConvert() was successful.
+// This return type informs the StrFormat extension framework (through
+// ADL but using the return type) of what conversion characters are supported.
+// It is strongly discouraged to return {false}, as this will result in an
+// empty string in StrFormat.
+template <FormatConversionCharSet C>
+struct FormatConvertResult {
+  bool value;
+};
+
 ABSL_NAMESPACE_END
 }  // namespace absl
 

diff --git a/absl/strings/str_format_test.cc b/absl/strings/str_format_test.cc
index 22cfef6..d9fb25a 100644
--- a/absl/strings/str_format_test.cc
+++ b/absl/strings/str_format_test.cc

@@ -16,7 +16,6 @@
 ABSL_NAMESPACE_BEGIN
 namespace {
 using str_format_internal::FormatArgImpl;
-using str_format_internal::FormatConversionCharSetInternal;
 
 using FormatEntryPointTest = ::testing::Test;
 
@@ -537,46 +536,90 @@
   EXPECT_FALSE((ParsedFormat<'s', 'd', 'g'>::New(format)));
 }
 
-using absl::str_format_internal::FormatConversionCharSet;
+#if defined(__cpp_nontype_template_parameter_auto)
+
+template <auto T>
+std::true_type IsValidParsedFormatArgTest(ParsedFormat<T>*);
+
+template <auto T>
+std::false_type IsValidParsedFormatArgTest(...);
+
+template <auto T>
+using IsValidParsedFormatArg = decltype(IsValidParsedFormatArgTest<T>(nullptr));
+
+TEST_F(ParsedFormatTest, OnlyValidTypesAllowed) {
+  ASSERT_TRUE(IsValidParsedFormatArg<'c'>::value);
+
+  ASSERT_TRUE(IsValidParsedFormatArg<FormatConversionCharSet::d>::value);
+
+  ASSERT_TRUE(IsValidParsedFormatArg<absl::FormatConversionCharSet::d |
+                                     absl::FormatConversionCharSet::x>::value);
+  ASSERT_TRUE(
+      IsValidParsedFormatArg<absl::FormatConversionCharSet::kIntegral>::value);
+
+  // This is an easy mistake to make, however, this will reduce to an integer
+  // which has no meaning, so we need to ensure it doesn't compile.
+  ASSERT_FALSE(IsValidParsedFormatArg<'x' | 'd'>::value);
+
+  // For now, we disallow construction based on ConversionChar (rather than
+  // CharSet)
+  ASSERT_FALSE(IsValidParsedFormatArg<absl::FormatConversionChar::d>::value);
+}
+
+TEST_F(ParsedFormatTest, ExtendedTyping) {
+  EXPECT_FALSE(ParsedFormat<FormatConversionCharSet::d>::New(""));
+  ASSERT_TRUE(ParsedFormat<absl::FormatConversionCharSet::d>::New("%d"));
+  auto v1 = ParsedFormat<'d', absl::FormatConversionCharSet::s>::New("%d%s");
+  ASSERT_TRUE(v1);
+  auto v2 = ParsedFormat<absl::FormatConversionCharSet::d, 's'>::New("%d%s");
+  ASSERT_TRUE(v2);
+  auto v3 = ParsedFormat<absl::FormatConversionCharSet::d |
+                             absl::FormatConversionCharSet::s,
+                         's'>::New("%d%s");
+  ASSERT_TRUE(v3);
+  auto v4 = ParsedFormat<absl::FormatConversionCharSet::d |
+                             absl::FormatConversionCharSet::s,
+                         's'>::New("%s%s");
+  ASSERT_TRUE(v4);
+}
+#endif
 
 TEST_F(ParsedFormatTest, UncheckedCorrect) {
   auto f =
-      ExtendedParsedFormat<FormatConversionCharSetInternal::d>::New("ABC%dDEF");
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New("ABC%dDEF");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]{d:1$d}[DEF]", SummarizeParsedFormat(*f));
 
   std::string format = "%sFFF%dZZZ%f";
   auto f2 = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::kString,
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::kFloating>::New(format);
+      absl::FormatConversionCharSet::kString, absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::kFloating>::New(format);
 
   ASSERT_TRUE(f2);
   EXPECT_EQ("{s:1$s}[FFF]{d:2$d}[ZZZ]{f:3$f}", SummarizeParsedFormat(*f2));
 
   f2 = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::kString,
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::kFloating>::New("%s %d %f");
+      absl::FormatConversionCharSet::kString, absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::kFloating>::New("%s %d %f");
 
   ASSERT_TRUE(f2);
   EXPECT_EQ("{s:1$s}[ ]{d:2$d}[ ]{f:3$f}", SummarizeParsedFormat(*f2));
 
   auto star =
-      ExtendedParsedFormat<FormatConversionCharSetInternal::kStar,
-                           FormatConversionCharSetInternal::d>::New("%*d");
+      ExtendedParsedFormat<absl::FormatConversionCharSet::kStar,
+                           absl::FormatConversionCharSet::d>::New("%*d");
   ASSERT_TRUE(star);
   EXPECT_EQ("{*d:2$1$*d}", SummarizeParsedFormat(*star));
 
-  auto dollar = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::s>::New("%2$s %1$d");
+  auto dollar =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                           absl::FormatConversionCharSet::s>::New("%2$s %1$d");
   ASSERT_TRUE(dollar);
   EXPECT_EQ("{2$s:2$s}[ ]{1$d:1$d}", SummarizeParsedFormat(*dollar));
   // with reuse
   dollar = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::s>::New("%2$s %1$d %1$d");
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::New("%2$s %1$d %1$d");
   ASSERT_TRUE(dollar);
   EXPECT_EQ("{2$s:2$s}[ ]{1$d:1$d}[ ]{1$d:1$d}",
             SummarizeParsedFormat(*dollar));
@@ -584,62 +627,61 @@
 
 TEST_F(ParsedFormatTest, UncheckedIgnoredArgs) {
   EXPECT_FALSE(
-      (ExtendedParsedFormat<FormatConversionCharSetInternal::d,
-                            FormatConversionCharSetInternal::s>::New("ABC")));
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("ABC")));
   EXPECT_FALSE(
-      (ExtendedParsedFormat<FormatConversionCharSetInternal::d,
-                            FormatConversionCharSetInternal::s>::New("%dABC")));
-  EXPECT_FALSE((ExtendedParsedFormat<
-                FormatConversionCharSetInternal::d,
-                FormatConversionCharSetInternal::s>::New("ABC%2$s")));
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("%dABC")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::s>::New("ABC%2$s")));
   auto f = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::s>::NewAllowIgnored("ABC");
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("ABC");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]", SummarizeParsedFormat(*f));
   f = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::s>::NewAllowIgnored("%dABC");
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("%dABC");
   ASSERT_TRUE(f);
   EXPECT_EQ("{d:1$d}[ABC]", SummarizeParsedFormat(*f));
   f = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d,
-      FormatConversionCharSetInternal::s>::NewAllowIgnored("ABC%2$s");
+      absl::FormatConversionCharSet::d,
+      absl::FormatConversionCharSet::s>::NewAllowIgnored("ABC%2$s");
   ASSERT_TRUE(f);
   EXPECT_EQ("[ABC]{2$s:2$s}", SummarizeParsedFormat(*f));
 }
 
 TEST_F(ParsedFormatTest, UncheckedMultipleTypes) {
-  auto dx = ExtendedParsedFormat<
-      FormatConversionCharSetInternal::d |
-      FormatConversionCharSetInternal::x>::New("%1$d %1$x");
+  auto dx =
+      ExtendedParsedFormat<absl::FormatConversionCharSet::d |
+                           absl::FormatConversionCharSet::x>::New("%1$d %1$x");
   EXPECT_TRUE(dx);
   EXPECT_EQ("{1$d:1$d}[ ]{1$x:1$x}", SummarizeParsedFormat(*dx));
 
-  dx = ExtendedParsedFormat<FormatConversionCharSetInternal::d |
-                            FormatConversionCharSetInternal::x>::New("%1$d");
+  dx = ExtendedParsedFormat<absl::FormatConversionCharSet::d |
+                            absl::FormatConversionCharSet::x>::New("%1$d");
   EXPECT_TRUE(dx);
   EXPECT_EQ("{1$d:1$d}", SummarizeParsedFormat(*dx));
 }
 
 TEST_F(ParsedFormatTest, UncheckedIncorrect) {
-  EXPECT_FALSE(
-      ExtendedParsedFormat<FormatConversionCharSetInternal::d>::New(""));
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New(""));
 
-  EXPECT_FALSE(ExtendedParsedFormat<FormatConversionCharSetInternal::d>::New(
+  EXPECT_FALSE(ExtendedParsedFormat<absl::FormatConversionCharSet::d>::New(
       "ABC%dDEF%d"));
 
   std::string format = "%sFFF%dZZZ%f";
   EXPECT_FALSE(
-      (ExtendedParsedFormat<FormatConversionCharSetInternal::s,
-                            FormatConversionCharSetInternal::d,
-                            FormatConversionCharSetInternal::g>::New(format)));
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::s,
+                            absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::g>::New(format)));
 }
 
 TEST_F(ParsedFormatTest, RegressionMixPositional) {
-  EXPECT_FALSE((ExtendedParsedFormat<
-                FormatConversionCharSetInternal::d,
-                FormatConversionCharSetInternal::o>::New("%1$d %o")));
+  EXPECT_FALSE(
+      (ExtendedParsedFormat<absl::FormatConversionCharSet::d,
+                            absl::FormatConversionCharSet::o>::New("%1$d %o")));
 }
 
 using FormatWrapperTest = ::testing::Test;
@@ -664,6 +706,38 @@
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+using FormatExtensionTest = ::testing::Test;
+
+struct Point {
+  friend absl::FormatConvertResult<absl::FormatConversionCharSet::kString |
+                                   absl::FormatConversionCharSet::kIntegral>
+  AbslFormatConvert(const Point& p, const absl::FormatConversionSpec& spec,
+                    absl::FormatSink* s) {
+    if (spec.conversion_char() == absl::FormatConversionChar::s) {
+      s->Append(absl::StrCat("x=", p.x, " y=", p.y));
+    } else {
+      s->Append(absl::StrCat(p.x, ",", p.y));
+    }
+    return {true};
+  }
+
+  int x = 10;
+  int y = 20;
+};
+
+TEST_F(FormatExtensionTest, AbslFormatConvertExample) {
+  Point p;
+  EXPECT_EQ(absl::StrFormat("a %s z", p), "a x=10 y=20 z");
+  EXPECT_EQ(absl::StrFormat("a %d z", p), "a 10,20 z");
+
+  // Typed formatting will fail to compile an invalid format.
+  // StrFormat("%f", p);  // Does not compile.
+  std::string actual;
+  absl::UntypedFormatSpec f1("%f");
+  // FormatUntyped will return false for bad character.
+  EXPECT_FALSE(absl::FormatUntyped(&actual, f1, {absl::FormatArg(p)}));
+}
+
 // Some codegen thunks that we can use to easily dump the generated assembly for
 // different StrFormat calls.
 

diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index a79cd4a..1ce17f3 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h

@@ -44,6 +44,7 @@
 #include <vector>
 
 #include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
 #include "absl/strings/internal/str_split_internal.h"
 #include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"

diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 1f8a696..62fa8e9 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc

@@ -39,6 +39,7 @@
 #include <thread>  // NOLINT(build/c++11)
 
 #include "absl/base/attributes.h"
+#include "absl/base/call_once.h"
 #include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/atomic_hook.h"
@@ -85,28 +86,6 @@
     kDeadlockDetectionDefault);
 ABSL_CONST_INIT std::atomic<bool> synch_check_invariants(false);
 
-// ------------------------------------------ spinlock support
-
-// Make sure read-only globals used in the Mutex code are contained on the
-// same cacheline and cacheline aligned to eliminate any false sharing with
-// other globals from this and other modules.
-static struct MutexGlobals {
-  MutexGlobals() {
-    // Find machine-specific data needed for Delay() and
-    // TryAcquireWithSpinning(). This runs in the global constructor
-    // sequence, and before that zeros are safe values.
-    num_cpus = absl::base_internal::NumCPUs();
-    spinloop_iterations = num_cpus > 1 ? 1500 : 0;
-  }
-  int num_cpus;
-  int spinloop_iterations;
-  // Pad this struct to a full cacheline to prevent false sharing.
-  char padding[ABSL_CACHELINE_SIZE - 2 * sizeof(int)];
-} ABSL_CACHELINE_ALIGNED mutex_globals;
-static_assert(
-    sizeof(MutexGlobals) == ABSL_CACHELINE_SIZE,
-    "MutexGlobals must occupy an entire cacheline to prevent false sharing");
-
 ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
     absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
         submit_profile_data;
@@ -143,7 +122,22 @@
   symbolizer.Store(fn);
 }
 
-// spinlock delay on iteration c.  Returns new c.
+struct ABSL_CACHELINE_ALIGNED MutexGlobals {
+  absl::once_flag once;
+  int num_cpus = 0;
+  int spinloop_iterations = 0;
+};
+
+static const MutexGlobals& GetMutexGlobals() {
+  ABSL_CONST_INIT static MutexGlobals data;
+  absl::base_internal::LowLevelCallOnce(&data.once, [&]() {
+    data.num_cpus = absl::base_internal::NumCPUs();
+    data.spinloop_iterations = data.num_cpus > 1 ? 1500 : 0;
+  });
+  return data;
+}
+
+// Spinlock delay on iteration c.  Returns new c.
 namespace {
   enum DelayMode { AGGRESSIVE, GENTLE };
 };
@@ -153,22 +147,25 @@
   // gentle then spin only a few times before yielding.  Aggressive spinning is
   // used to ensure that an Unlock() call, which  must get the spin lock for
   // any thread to make progress gets it without undue delay.
-  int32_t limit = (mutex_globals.num_cpus > 1) ?
-      ((mode == AGGRESSIVE) ? 5000 : 250) : 0;
+  const int32_t limit =
+      GetMutexGlobals().num_cpus > 1 ? (mode == AGGRESSIVE ? 5000 : 250) : 0;
   if (c < limit) {
-    c++;               // spin
+    // Spin.
+    c++;
   } else {
     ABSL_TSAN_MUTEX_PRE_DIVERT(nullptr, 0);
-    if (c == limit) {  // yield once
+    if (c == limit) {
+      // Yield once.
       AbslInternalMutexYield();
       c++;
-    } else {           // then wait
+    } else {
+      // Then wait.
       absl::SleepFor(absl::Microseconds(10));
       c = 0;
     }
     ABSL_TSAN_MUTEX_POST_DIVERT(nullptr, 0);
   }
-  return (c);
+  return c;
 }
 
 // --------------------------Generic atomic ops
@@ -1437,7 +1434,7 @@
 // Attempt to acquire *mu, and return whether successful.  The implementation
 // 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 c = GetMutexGlobals().spinloop_iterations;
   do {  // do/while somewhat faster on AMD
     intptr_t v = mu->load(std::memory_order_relaxed);
     if ((v & (kMuReader|kMuEvent)) != 0) {

diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index d0f1aad..952cc09 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc

@@ -69,6 +69,7 @@
 #include "absl/base/casts.h"
 #include "absl/base/macros.h"
 #include "absl/numeric/int128.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 #include "absl/time/time.h"
 
@@ -710,16 +711,17 @@
 // fractional digits, because it is in the noise of what a Duration can
 // represent.
 struct DisplayUnit {
-  const char* abbr;
+  absl::string_view abbr;
   int prec;
   double pow10;
 };
-const DisplayUnit kDisplayNano = {"ns", 2, 1e2};
-const DisplayUnit kDisplayMicro = {"us", 5, 1e5};
-const DisplayUnit kDisplayMilli = {"ms", 8, 1e8};
-const DisplayUnit kDisplaySec = {"s", 11, 1e11};
-const DisplayUnit kDisplayMin = {"m", -1, 0.0};   // prec ignored
-const DisplayUnit kDisplayHour = {"h", -1, 0.0};  // prec ignored
+ABSL_CONST_INIT const DisplayUnit kDisplayNano = {"ns", 2, 1e2};
+ABSL_CONST_INIT const DisplayUnit kDisplayMicro = {"us", 5, 1e5};
+ABSL_CONST_INIT const DisplayUnit kDisplayMilli = {"ms", 8, 1e8};
+ABSL_CONST_INIT const DisplayUnit kDisplaySec = {"s", 11, 1e11};
+ABSL_CONST_INIT const DisplayUnit kDisplayMin = {"m", -1, 0.0};  // prec ignored
+ABSL_CONST_INIT const DisplayUnit kDisplayHour = {"h", -1,
+                                                  0.0};  // prec ignored
 
 void AppendNumberUnit(std::string* out, int64_t n, DisplayUnit unit) {
   char buf[sizeof("2562047788015216")];  // hours in max duration
@@ -727,7 +729,7 @@
   char* bp = Format64(ep, 0, n);
   if (*bp != '0' || bp + 1 != ep) {
     out->append(bp, ep - bp);
-    out->append(unit.abbr);
+    out->append(unit.abbr.data(), unit.abbr.size());
   }
 }
 
@@ -750,7 +752,7 @@
       while (ep[-1] == '0') --ep;
       out->append(bp, ep - bp);
     }
-    out->append(unit.abbr);
+    out->append(unit.abbr.data(), unit.abbr.size());
   }
 }
 

diff --git a/absl/time/internal/cctz/include/cctz/civil_time_detail.h b/absl/time/internal/cctz/include/cctz/civil_time_detail.h
index 4cde96f..d1b4222 100644
--- a/absl/time/internal/cctz/include/cctz/civil_time_detail.h
+++ b/absl/time/internal/cctz/include/cctz/civil_time_detail.h

@@ -106,54 +106,64 @@
 
 CONSTEXPR_F fields n_day(year_t y, month_t m, diff_t d, diff_t cd, hour_t hh,
                          minute_t mm, second_t ss) noexcept {
-  y += (cd / 146097) * 400;
+  year_t ey = y % 400;
+  const year_t oey = ey;
+  ey += (cd / 146097) * 400;
   cd %= 146097;
   if (cd < 0) {
-    y -= 400;
+    ey -= 400;
     cd += 146097;
   }
-  y += (d / 146097) * 400;
+  ey += (d / 146097) * 400;
   d = d % 146097 + cd;
   if (d > 0) {
     if (d > 146097) {
-      y += 400;
+      ey += 400;
       d -= 146097;
     }
   } else {
     if (d > -365) {
       // We often hit the previous year when stepping a civil time backwards,
       // so special case it to avoid counting up by 100/4/1-year chunks.
-      y -= 1;
-      d += days_per_year(y, m);
+      ey -= 1;
+      d += days_per_year(ey, m);
     } else {
-      y -= 400;
+      ey -= 400;
       d += 146097;
     }
   }
   if (d > 365) {
-    for (int n = days_per_century(y, m); d > n; n = days_per_century(y, m)) {
+    for (;;) {
+      int n = days_per_century(ey, m);
+      if (d <= n) break;
       d -= n;
-      y += 100;
+      ey += 100;
     }
-    for (int n = days_per_4years(y, m); d > n; n = days_per_4years(y, m)) {
+    for (;;) {
+      int n = days_per_4years(ey, m);
+      if (d <= n) break;
       d -= n;
-      y += 4;
+      ey += 4;
     }
-    for (int n = days_per_year(y, m); d > n; n = days_per_year(y, m)) {
+    for (;;) {
+      int n = days_per_year(ey, m);
+      if (d <= n) break;
       d -= n;
-      ++y;
+      ++ey;
     }
   }
   if (d > 28) {
-    for (int n = days_per_month(y, m); d > n; n = days_per_month(y, m)) {
+    for (;;) {
+      int n = days_per_month(ey, m);
+      if (d <= n) break;
       d -= n;
       if (++m > 12) {
-        ++y;
+        ++ey;
         m = 1;
       }
     }
   }
-  return fields(y, m, static_cast<day_t>(d), hh, mm, ss);
+  return fields(y + (ey - oey), m, static_cast<day_t>(d), hh, mm, ss);
 }
 CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd, hour_t hh,
                          minute_t mm, second_t ss) noexcept {

diff --git a/absl/time/internal/cctz/src/civil_time_test.cc b/absl/time/internal/cctz/src/civil_time_test.cc
index be894d7..a5a7123 100644
--- a/absl/time/internal/cctz/src/civil_time_test.cc
+++ b/absl/time/internal/cctz/src/civil_time_test.cc

@@ -235,6 +235,16 @@
 }
 
 // NOTE: Run this with --copt=-ftrapv to detect overflow problems.
+TEST(CivilTime, ConstructionWithHugeYear) {
+  constexpr civil_hour h(-9223372036854775807, 1, 1, -1);
+  static_assert(h.year() == -9223372036854775807 - 1,
+                "ConstructionWithHugeYear");
+  static_assert(h.month() == 12, "ConstructionWithHugeYear");
+  static_assert(h.day() == 31, "ConstructionWithHugeYear");
+  static_assert(h.hour() == 23, "ConstructionWithHugeYear");
+}
+
+// NOTE: Run this with --copt=-ftrapv to detect overflow problems.
 TEST(CivilTime, DifferenceWithHugeYear) {
   {
     constexpr civil_day d1(9223372036854775807, 1, 1);

diff --git a/absl/types/internal/variant.h b/absl/types/internal/variant.h
index 71bd3ad..d404e80 100644
--- a/absl/types/internal/variant.h
+++ b/absl/types/internal/variant.h

@@ -292,7 +292,7 @@
 template <class Op, std::size_t I>
 struct ReachableSwitchCase {
   static VisitIndicesResultT<Op, std::size_t> Run(Op&& op) {
-    return absl::base_internal::Invoke(absl::forward<Op>(op), SizeT<I>());
+    return absl::base_internal::invoke(absl::forward<Op>(op), SizeT<I>());
   }
 };
 
@@ -424,7 +424,7 @@
         return PickCase<Op, 32, EndIndex>::Run(absl::forward<Op>(op));
       default:
         ABSL_ASSERT(i == variant_npos);
-        return absl::base_internal::Invoke(absl::forward<Op>(op), NPos());
+        return absl::base_internal::invoke(absl::forward<Op>(op), NPos());
     }
   }
 };
@@ -488,7 +488,7 @@
     template <std::size_t I>
     VisitIndicesResultT<Op, decltype(EndIndices)...> operator()(
         SizeT<I> /*index*/) && {
-      return base_internal::Invoke(
+      return base_internal::invoke(
           absl::forward<Op>(op),
           SizeT<UnflattenIndex<I, N, (EndIndices + 1)...>::value -
                 std::size_t{1}>()...);
@@ -930,7 +930,7 @@
                      absl::result_of_t<Op(VariantAccessResult<
                                           Is, QualifiedVariants>...)>>::value,
         "All visitation overloads must have the same return type.");
-    return absl::base_internal::Invoke(
+    return absl::base_internal::invoke(
         absl::forward<Op>(op),
         VariantCoreAccess::Access<Is>(
             absl::forward<QualifiedVariants>(std::get<TupIs>(variant_tup)))...);

diff --git a/absl/types/span.h b/absl/types/span.h
index 734db69..95fe792 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h

@@ -17,32 +17,30 @@
 // span.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines a `Span<T>` type for holding a view of an existing
-// array of data. The `Span` object, much like the `absl::string_view` object,
-// does not own such data itself. A span provides a lightweight way to pass
-// around view of such data.
+// This header file defines a `Span<T>` type for holding a reference to existing
+// array data. The `Span` object, much like the `absl::string_view` object,
+// does not own such data itself, and the data being referenced by the span must
+// outlive the span itself. Unlike `view` type references, a span can hold a
+// reference to mutable data (and can mutate it for underlying types of
+// non-const T.) A span provides a lightweight way to pass a reference to such
+// data.
 //
 // Additionally, this header file defines `MakeSpan()` and `MakeConstSpan()`
 // factory functions, for clearly creating spans of type `Span<T>` or read-only
 // `Span<const T>` when such types may be difficult to identify due to issues
 // with implicit conversion.
 //
-// The C++ standards committee currently has a proposal for a `std::span` type,
-// (http://wg21.link/p0122), which is not yet part of the standard (though may
-// become part of C++20). As of August 2017, the differences between
-// `absl::Span` and this proposal are:
-//    * `absl::Span` uses `size_t` for `size_type`
-//    * `absl::Span` has no `operator()`
-//    * `absl::Span` has no constructors for `std::unique_ptr` or
-//      `std::shared_ptr`
+// The C++20 draft standard includes a `std::span` type. As of June 2020, the
+// differences between `absl::Span` and `std::span` are:
+//    * `absl::Span` has `operator==` (which is likely a design bug,
+//       per https://abseil.io/blog/20180531-regular-types)
 //    * `absl::Span` has the factory functions `MakeSpan()` and
 //      `MakeConstSpan()`
-//    * `absl::Span` has `front()` and `back()` methods
 //    * bounds-checked access to `absl::Span` is accomplished with `at()`
 //    * `absl::Span` has compiler-provided move and copy constructors and
 //      assignment. This is due to them being specified as `constexpr`, but that
 //      implies const in C++11.
-//    * `absl::Span` has no `element_type` or `index_type` typedefs
+//    * `absl::Span` has no `element_type` typedef
 //    * A read-only `absl::Span<const T>` can be implicitly constructed from an
 //      initializer list.
 //    * `absl::Span` has no `bytes()`, `size_bytes()`, `as_bytes()`, or
@@ -77,9 +75,9 @@
 // Span
 //------------------------------------------------------------------------------
 //
-// A `Span` is an "array view" type for holding a view of a contiguous data
-// array; the `Span` object does not and cannot own such data itself. A span
-// provides an easy way to provide overloads for anything operating on
+// A `Span` is an "array reference" type for holding a reference of contiguous
+// array data; the `Span` object does not and cannot own such data itself. A
+// span provides an easy way to provide overloads for anything operating on
 // contiguous sequences without needing to manage pointers and array lengths
 // manually.
 
@@ -97,7 +95,8 @@
 // constructors.
 //
 // A `Span<T>` is somewhat analogous to an `absl::string_view`, but for an array
-// of elements of type `T`. A user of `Span` must ensure that the data being
+// of elements of type `T`, and unlike an `absl::string_view`, a span can hold a
+// reference to mutable data. A user of `Span` must ensure that the data being
 // pointed to outlives the `Span` itself.
 //
 // You can construct a `Span<T>` in several ways:
@@ -127,7 +126,7 @@
 // Note that `Span` objects, in addition to requiring that the memory they
 // point to remains alive, must also ensure that such memory does not get
 // reallocated. Therefore, to avoid undefined behavior, containers with
-// associated span views should not invoke operations that may reallocate memory
+// associated spans should not invoke operations that may reallocate memory
 // (such as resizing) or invalidate iterators into the container.
 //
 // One common use for a `Span` is when passing arguments to a routine that can

diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index e6647c7..bf92322 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h

@@ -236,10 +236,10 @@
 // Helper method for expanding tuple into a called method.
 template <typename Functor, typename Tuple, std::size_t... Indexes>
 auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
-    -> decltype(absl::base_internal::Invoke(
+    -> decltype(absl::base_internal::invoke(
         absl::forward<Functor>(functor),
         std::get<Indexes>(absl::forward<Tuple>(t))...)) {
-  return absl::base_internal::Invoke(
+  return absl::base_internal::invoke(
       absl::forward<Functor>(functor),
       std::get<Indexes>(absl::forward<Tuple>(t))...);
 }