base/functional/bind_internal.h - chromium/src - Git at Google

 // Copyright 2011 The Chromium Authors
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #ifndef BASE_FUNCTIONAL_BIND_INTERNAL_H_
 #define BASE_FUNCTIONAL_BIND_INTERNAL_H_

 #include <stddef.h>

 #include <concepts>
 #include <functional>
 #include <memory>
 #include <tuple>
 #include <type_traits>
 #include <utility>

 #include "base/check.h"
 #include "base/compiler_specific.h"
 #include "base/functional/callback_internal.h"
 #include "base/functional/unretained_traits.h"
 #include "base/memory/raw_ptr.h"
 #include "base/memory/raw_ref.h"
 #include "base/memory/weak_ptr.h"
 #include "base/types/is_complete.h"
 #include "base/types/is_instantiation.h"
 #include "base/types/to_address.h"
 #include "build/build_config.h"
 #include "third_party/abseil-cpp/absl/functional/function_ref.h"

 #if PA_BUILDFLAG(USE_ASAN_BACKUP_REF_PTR)
 #include "base/memory/raw_ptr_asan_bound_arg_tracker.h"
 #endif

 // See docs/callback.md for user documentation.
 //
 // Concepts:
 //  Functor -- A movable type representing something that should be called.
 //             All function pointers and `Callback<>` are functors even if the
 //             invocation syntax differs.
 //  RunType -- A function type (as opposed to function _pointer_ type) for
 //             a `Callback<>::Run()`.  Usually just a convenience typedef.
 //  (Bound)Args -- A set of types that stores the arguments.
 //
 // Types:
 //  `ForceVoidReturn<>` -- Helper class for translating function signatures to
 //                         equivalent forms with a `void` return type.
 //  `FunctorTraits<>` -- Type traits used to determine the correct RunType and
 //                       invocation manner for a Functor.  This is where
 //                       function signature adapters are applied.
 //  `StorageTraits<>` -- Type traits that determine how a bound argument is
 //                       stored in `BindState<>`.
 //  `InvokeHelper<>` -- Takes a Functor + arguments and actually invokes it.
 //                      Handles the differing syntaxes needed for `WeakPtr<>`
 //                      support.  This is separate from `Invoker<>` to avoid
 //                      creating multiple versions of `Invoker<>`.
 //  `Invoker<>` -- Unwraps the curried parameters and executes the Functor.
 //  `BindState<>` -- Stores the curried parameters, and is the main entry point
 //                   into the `Bind()` system.

 #if BUILDFLAG(IS_WIN)
 namespace Microsoft {
 namespace WRL {
 template <typename>
 class ComPtr;
 }  // namespace WRL
 }  // namespace Microsoft
 #endif

 namespace base {

 template <typename T>
 struct IsWeakReceiver;

 template <typename>
 struct BindUnwrapTraits;

 template <typename Functor, typename BoundArgsTuple>
 struct CallbackCancellationTraits;

 template <typename Signature>
 class FunctionRef;

 // A tag type to return when `Bind()` calls fail. In this case we intentionally
 // don't return `void`, since that would produce spurious errors like "variable
 // has incomplete type 'void'" when assigning the result of
 // `Bind{Once,Repeating}()` to an `auto`.
 struct BindFailedCheckPreviousErrors {};

 namespace unretained_traits {

 // `UnretainedWrapper` will check and report if pointer is dangling upon
 // invocation.
 struct MayNotDangle {};
 // `UnretainedWrapper` won't check if pointer is dangling upon invocation. For
 // extra safety, the receiver must be of type `MayBeDangling<>`.
 struct MayDangle {};
 // `UnretainedWrapper` won't check if pointer is dangling upon invocation. The
 // receiver doesn't have to be a `raw_ptr<>`. This is just a temporary state, to
 // allow dangling pointers that would otherwise crash if `MayNotDangle` was
 // used. It should be replaced ASAP with `MayNotDangle` (after fixing the
 // dangling pointers) or with `MayDangle` if there is really no other way (after
 // making receivers `MayBeDangling<>`).
 struct MayDangleUntriaged {};

 }  // namespace unretained_traits

 namespace internal {

 template <typename T,
           typename UnretainedTrait,
           RawPtrTraits PtrTraits = RawPtrTraits::kEmpty>
 class UnretainedWrapper {
   // Note that if `PtrTraits` already includes `MayDangle`, `DanglingRawPtrType`
   // will be identical to `raw_ptr<T, PtrTraits>`.
   using DanglingRawPtrType = MayBeDangling<T, PtrTraits>;

  public:
   // We want the getter type to match the receiver parameter that it is passed
   // into, to minimize `raw_ptr<T>` <-> `T*` conversions. We also would like to
   // match `StorageType`, but sometimes we can't have both, as shown in
   // https://docs.google.com/document/d/1dLM34aKqbNBfRdOYxxV_T-zQU4J5wjmXwIBJZr7JvZM/edit
   // When we can't have both, prefer the former, mostly because
   // `GetPtrType`=`raw_ptr<T>` would break if e.g. `UnretainedWrapper()` is
   // constructed using `char*`, but the receiver is of type `std::string&`.
   // This is enforced by `static_assert()`s in `ParamCanBeBound`.
   using GetPtrType = std::conditional_t<
       raw_ptr_traits::IsSupportedType<T>::value &&
           std::same_as<UnretainedTrait, unretained_traits::MayDangle>,
       DanglingRawPtrType,
       T*>;

   // Raw pointer makes sense only if there are no `PtrTrait`s. If there are,
   // it means that a `raw_ptr` is being passed, so use the ctors below instead.
   explicit UnretainedWrapper(T* o)
     requires(PtrTraits == RawPtrTraits::kEmpty)
       : ptr_(o) {
     VerifyPreconditions();
   }

   explicit UnretainedWrapper(const raw_ptr<T, PtrTraits>& o)
     requires(raw_ptr_traits::IsSupportedType<T>::value)
       : ptr_(o) {
     VerifyPreconditions();
   }

   explicit UnretainedWrapper(raw_ptr<T, PtrTraits>&& o)
     requires(raw_ptr_traits::IsSupportedType<T>::value)
       : ptr_(std::move(o)) {
     VerifyPreconditions();
   }

   GetPtrType get() const { return GetInternal(ptr_); }

   // True if this type is valid. When this is false, a `static_assert` will have
   // been fired explaining why.
   static constexpr bool value = SupportsUnretained<T>;

  private:
   // `ptr_` is either a `raw_ptr` or a regular C++ pointer.
   template <typename U>
     requires std::same_as<T, U>
   static GetPtrType GetInternal(U* ptr) {
     return ptr;
   }
   template <typename U, RawPtrTraits Traits>
     requires std::same_as<T, U>
   static GetPtrType GetInternal(const raw_ptr<U, Traits>& ptr) {
     if constexpr (std::same_as<UnretainedTrait,
                                unretained_traits::MayNotDangle>) {
       ptr.ReportIfDangling();
     }
     return ptr;
   }

   // `Unretained()` arguments often dangle by design (a common design pattern
   // is to manage an object's lifetime inside the callback itself, using
   // stateful information), so disable direct dangling pointer detection
   // of `ptr_`.
   //
   // If the callback is invoked, dangling pointer detection will be triggered
   // before invoking the bound functor (unless stated otherwise, see
   // `UnsafeDangling()` and `UnsafeDanglingUntriaged()`), when retrieving the
   // pointer value via `get()` above.
   using StorageType =
       std::conditional_t<raw_ptr_traits::IsSupportedType<T>::value,
                          DanglingRawPtrType,
                          T*>;
   // Avoid converting between different `raw_ptr` types when calling `get()`.
   // It is allowable to convert `raw_ptr<T>` -> `T*`, but not in the other
   // direction. See the comment by `GetPtrType` describing for more details.
   static_assert(std::is_pointer_v<GetPtrType> ||
                 std::same_as<GetPtrType, StorageType>);

   // Forces `value` to be materialized, performing a compile-time check of the
   // preconditions if it hasn't already occurred. This is called from every
   // constructor so the wrappers in bind.h don't have to each check it, and so
   // no one can go around them and construct this underlying type directly.
   static constexpr void VerifyPreconditions() {
     // Using `static_assert(value);` here would work but fire an extra error.
     std::ignore = value;
   }

   StorageType ptr_;
 };

 // Storage type for `std::reference_wrapper` so `BindState` can internally store
 // unprotected references using `raw_ref`.
 //
 // `std::reference_wrapper<T>` and `T&` do not work, since the reference
 // lifetime is not safely protected by MiraclePtr.
 //
 // `UnretainedWrapper<T>` and `raw_ptr<T>` do not work, since `BindUnwrapTraits`
 // would try to pass by `T*` rather than `T&`.
 template <typename T,
           typename UnretainedTrait,
           RawPtrTraits PtrTraits = RawPtrTraits::kEmpty>
 class UnretainedRefWrapper {
  public:
   // Raw reference makes sense only if there are no `PtrTrait`s. If there are,
   // it means that a `raw_ref` is being passed, so use the ctors below instead.
   explicit UnretainedRefWrapper(T& o)
     requires(PtrTraits == RawPtrTraits::kEmpty)
       : ref_(o) {
     VerifyPreconditions();
   }

   explicit UnretainedRefWrapper(const raw_ref<T, PtrTraits>& o)
     requires(raw_ptr_traits::IsSupportedType<T>::value)
       : ref_(o) {
     VerifyPreconditions();
   }

   explicit UnretainedRefWrapper(raw_ref<T, PtrTraits>&& o)
     requires(raw_ptr_traits::IsSupportedType<T>::value)
       : ref_(std::move(o)) {
     VerifyPreconditions();
   }

   T& get() const { return GetInternal(ref_); }

   // See comments in `UnretainedWrapper` regarding this and
   // `VerifyPreconditions()`.
   static constexpr bool value = SupportsUnretained<T>;

  private:
   // `ref_` is either a `raw_ref` or a regular C++ reference.
   template <typename U>
     requires std::same_as<T, U>
   static T& GetInternal(U& ref) {
     return ref;
   }
   template <typename U, RawPtrTraits Traits>
     requires std::same_as<T, U>
   static T& GetInternal(const raw_ref<U, Traits>& ref) {
     // The ultimate goal is to crash when a callback is invoked with a
     // dangling pointer. This is checked here. For now, it is configured to
     // either crash, DumpWithoutCrashing or be ignored. This depends on the
     // `PartitionAllocUnretainedDanglingPtr` feature.
     if constexpr (std::is_same_v<UnretainedTrait,
                                  unretained_traits::MayNotDangle>) {
       ref.ReportIfDangling();
     }
     // We can't use `operator*` here, we need to use `raw_ptr`'s
     // `GetForExtraction` instead of `GetForDereference`. If we did use
     // `GetForDereference` then we'd crash in ASAN builds on calling a bound
     // callback with a dangling reference parameter even if that parameter is
     // not used. This could hide a later unprotected issue that would be reached
     // in release builds.
     return ref.get();
   }

   // `Unretained()` arguments often dangle by design (a common design pattern
   // is to manage an object's lifetime inside the callback itself, using
   // stateful information), so disable direct dangling pointer detection
   // of `ref_`.
   //
   // If the callback is invoked, dangling pointer detection will be triggered
   // before invoking the bound functor (unless stated otherwise, see
   // `UnsafeDangling()` and `UnsafeDanglingUntriaged()`), when retrieving the
   // pointer value via `get()` above.
   using StorageType =
       std::conditional_t<raw_ptr_traits::IsSupportedType<T>::value,
                          raw_ref<T, DisableDanglingPtrDetection>,
                          T&>;

   static constexpr void VerifyPreconditions() { std::ignore = value; }

   StorageType ref_;
 };

 // Can't use `is_instantiation` to detect the unretained wrappers, since they
 // have non-type template params.
 template <template <typename, typename, RawPtrTraits> typename WrapperT,
           typename T>
 inline constexpr bool kIsUnretainedWrapper = false;

 template <template <typename, typename, RawPtrTraits> typename WrapperT,
           typename T,
           typename UnretainedTrait,
           RawPtrTraits PtrTraits>
 inline constexpr bool
     kIsUnretainedWrapper<WrapperT, WrapperT<T, UnretainedTrait, PtrTraits>> =
         true;

 // The class is used to wrap `UnretainedRefWrapper` when the latter is used as
 // a method receiver (a reference on `this` argument). This is needed because
 // the internal callback mechanism expects the receiver to have the type
 // `MyClass*` and to have `operator*`.
 // This is used as storage.
 template <typename T, typename UnretainedTrait, RawPtrTraits PtrTraits>
 class UnretainedRefWrapperReceiver {
  public:
   // NOLINTNEXTLINE(google-explicit-constructor)
   UnretainedRefWrapperReceiver(
       UnretainedRefWrapper<T, UnretainedTrait, PtrTraits>&& obj)
       : obj_(std::move(obj)) {}

   T& operator*() const { return obj_.get(); }
   T* operator->() const { return &obj_.get(); }

  private:
   UnretainedRefWrapper<T, UnretainedTrait, PtrTraits> obj_;
 };

 // `MethodReceiverStorage` converts the current receiver type to its stored
 // type. For instance, it converts pointers to `scoped_refptr`, and wraps
 // `UnretainedRefWrapper` to make it compliant with the internal callback
 // invocation mechanism.
 template <typename T>
 struct MethodReceiverStorage {
   using Type = std::
       conditional_t<IsPointerOrRawPtr<T>, scoped_refptr<RemovePointerT<T>>, T>;
 };

 template <typename T, typename UnretainedTrait, RawPtrTraits PtrTraits>
 struct MethodReceiverStorage<
     UnretainedRefWrapper<T, UnretainedTrait, PtrTraits>> {
   // We can't use `UnretainedRefWrapper` as a receiver directly (see
   // `UnretainedRefWrapperReceiver` for why).
   using Type = UnretainedRefWrapperReceiver<T, UnretainedTrait, PtrTraits>;
 };

 template <typename T>
 class RetainedRefWrapper {
  public:
   explicit RetainedRefWrapper(T* o) : ptr_(o) {}
   explicit RetainedRefWrapper(scoped_refptr<T> o) : ptr_(std::move(o)) {}
   T* get() const { return ptr_.get(); }

  private:
   scoped_refptr<T> ptr_;
 };

 template <typename T>
 struct IgnoreResultHelper {
   explicit IgnoreResultHelper(T functor) : functor_(std::move(functor)) {}
   explicit operator bool() const { return !!functor_; }

   T functor_;
 };

 template <typename T, typename Deleter = std::default_delete<T>>
 class OwnedWrapper {
  public:
   explicit OwnedWrapper(T* o) : ptr_(o) {}
   explicit OwnedWrapper(std::unique_ptr<T, Deleter>&& ptr)
       : ptr_(std::move(ptr)) {}
   T* get() const { return ptr_.get(); }

  private:
   std::unique_ptr<T, Deleter> ptr_;
 };

 template <typename T>
 class OwnedRefWrapper {
  public:
   explicit OwnedRefWrapper(const T& t) : t_(t) {}
   explicit OwnedRefWrapper(T&& t) : t_(std::move(t)) {}
   T& get() const { return t_; }

  private:
   mutable T t_;
 };

 // `PassedWrapper` is a copyable adapter for a scoper that ignores `const`.
 //
 // It is needed to get around the fact that `Bind()` takes a const reference to
 // all its arguments.  Because `Bind()` takes a const reference to avoid
 // unnecessary copies, it is incompatible with movable-but-not-copyable
 // types; doing a destructive "move" of the type into `Bind()` would violate
 // the const correctness.
 //
 // This conundrum cannot be solved without either rvalue references or an O(2^n)
 // blowup of `Bind()` templates to handle each combination of regular types and
 // movable-but-not-copyable types.  Thus we introduce a wrapper type that is
 // copyable to transmit the correct type information down into `BindState<>`.
 // Ignoring `const` in this type makes sense because it is only created when we
 // are explicitly trying to do a destructive move.
 //
 // Two notes:
 //  1) `PassedWrapper` supports any type that has a move constructor, however
 //     the type will need to be specifically allowed in order for it to be
 //     bound to a `Callback`. We guard this explicitly at the call of `Passed()`
 //     to make for clear errors. Things not given to `Passed()` will be
 //     forwarded and stored by value which will not work for general move-only
 //     types.
 //  2) `is_valid_` is distinct from `nullptr` because it is valid to bind a null
 //     scoper to a `Callback` and allow the `Callback` to execute once.
 //
 // TODO(crbug.com/40840557): We have rvalue references and such now. Remove.
 template <typename T>
 class PassedWrapper {
  public:
   explicit PassedWrapper(T&& scoper) : scoper_(std::move(scoper)) {}
   PassedWrapper(PassedWrapper&& other)
       : is_valid_(other.is_valid_), scoper_(std::move(other.scoper_)) {}
   T Take() const {
     CHECK(is_valid_);
     is_valid_ = false;
     return std::move(scoper_);
   }

  private:
   mutable bool is_valid_ = true;
   mutable T scoper_;
 };

 template <typename T>
 using Unwrapper = BindUnwrapTraits<std::decay_t<T>>;

 template <typename T>
 decltype(auto) Unwrap(T&& o) {
   return Unwrapper<T>::Unwrap(std::forward<T>(o));
 }

 // `kIsWeakMethod` is a helper that determines if we are binding a `WeakPtr<>`
 // to a method. It is used internally by `Bind()` to select the correct
 // `InvokeHelper` that will no-op itself in the event the `WeakPtr<>` for the
 // target object is invalidated.
 //
 // The first argument should be the type of the object that will be received by
 // the method.
 template <bool is_method, typename... Args>
 inline constexpr bool kIsWeakMethod = false;

 template <typename T, typename... Args>
 inline constexpr bool kIsWeakMethod<true, T, Args...> =
     IsWeakReceiver<T>::value;

 // Packs a list of types to hold them in a single type.
 template <typename... Types>
 struct TypeList {};

 // Implements `DropTypeListItem`.
 template <size_t n, typename List>
   requires is_instantiation<List, TypeList>
 struct DropTypeListItemImpl {
   using Type = List;
 };

 template <size_t n, typename T, typename... List>
   requires(n > 0)
 struct DropTypeListItemImpl<n, TypeList<T, List...>>
     : DropTypeListItemImpl<n - 1, TypeList<List...>> {};

 // A type-level function that drops `n` list items from a given `TypeList`.
 template <size_t n, typename List>
 using DropTypeListItem = typename DropTypeListItemImpl<n, List>::Type;

 // Implements `TakeTypeListItem`.
 template <size_t n, typename List, typename... Accum>
   requires is_instantiation<List, TypeList>
 struct TakeTypeListItemImpl {
   using Type = TypeList<Accum...>;
 };

 template <size_t n, typename T, typename... List, typename... Accum>
   requires(n > 0)
 struct TakeTypeListItemImpl<n, TypeList<T, List...>, Accum...>
     : TakeTypeListItemImpl<n - 1, TypeList<List...>, Accum..., T> {};

 // A type-level function that takes the first `n` items from a given `TypeList`;
 // e.g. `TakeTypeListItem<3, TypeList<A, B, C, D>>` -> `TypeList<A, B, C>`.
 template <size_t n, typename List>
 using TakeTypeListItem = typename TakeTypeListItemImpl<n, List>::Type;

 // Implements `MakeFunctionType`.
 template <typename R, typename ArgList>
 struct MakeFunctionTypeImpl;

 template <typename R, typename... Args>
 struct MakeFunctionTypeImpl<R, TypeList<Args...>> {
   using Type = R(Args...);
 };

 // A type-level function that constructs a function type that has `R` as its
 // return type and has a `TypeList`'s items as its arguments.
 template <typename R, typename ArgList>
 using MakeFunctionType = typename MakeFunctionTypeImpl<R, ArgList>::Type;

 // Implements `ExtractArgs` and `ExtractReturnType`.
 template <typename Signature>
 struct ExtractArgsImpl;

 template <typename R, typename... Args>
 struct ExtractArgsImpl<R(Args...)> {
   using ReturnType = R;
   using ArgsList = TypeList<Args...>;
 };

 // A type-level function that extracts function arguments into a `TypeList`;
 // e.g. `ExtractArgs<R(A, B, C)>` -> `TypeList<A, B, C>`.
 template <typename Signature>
 using ExtractArgs = typename ExtractArgsImpl<Signature>::ArgsList;

 // A type-level function that extracts the return type of a function.
 // e.g. `ExtractReturnType<R(A, B, C)>` -> `R`.
 template <typename Signature>
 using ExtractReturnType = typename ExtractArgsImpl<Signature>::ReturnType;

 template <typename Callable,
           typename Signature = decltype(&Callable::operator())>
 struct ExtractCallableRunTypeImpl;

 #define BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS(quals)     \
   template <typename Callable, typename R, typename... Args>          \
   struct ExtractCallableRunTypeImpl<Callable,                         \
                                     R (Callable::*)(Args...) quals> { \
     using Type = R(Args...);                                          \
   }

 BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS();
 BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS(const);
 BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS(noexcept);
 BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS(const noexcept);

 #undef BIND_INTERNAL_EXTRACT_CALLABLE_RUN_TYPE_WITH_QUALS

 // Evaluated to the RunType of the given callable type; e.g.
 // `ExtractCallableRunType<decltype([](int, char*) { return 0.1; })>` ->
 //     `double(int, char*)`.
 template <typename Callable>
 using ExtractCallableRunType =
     typename ExtractCallableRunTypeImpl<Callable>::Type;

 // True when `Functor` has a non-overloaded `operator()()`, e.g.:
 //   struct S1 {
 //     int operator()(int);
 //   };
 //   static_assert(HasNonOverloadedCallOp<S1>);
 //
 //   int i = 0;
 //   auto f = [i] {};
 //   static_assert(HasNonOverloadedCallOp<decltype(f)>);
 //
 //   struct S2 {
 //     int operator()(int);
 //     std::string operator()(std::string);
 //   };
 //   static_assert(!HasNonOverloadedCallOp<S2>);
 //
 //   static_assert(!HasNonOverloadedCallOp<void(*)()>);
 //
 //   struct S3 {};
 //   static_assert(!HasNonOverloadedCallOp<S3>);
 // ```
 template <typename Functor>
 concept HasNonOverloadedCallOp = requires { &Functor::operator(); };

 template <typename T>
 inline constexpr bool IsObjCArcBlockPointer = false;

 #if __OBJC__ && HAS_FEATURE(objc_arc)
 template <typename R, typename... Args>
 inline constexpr bool IsObjCArcBlockPointer<R (^)(Args...)> = true;
 #endif

 // True when `Functor` has an overloaded `operator()()` that can be invoked with
 // the provided `BoundArgs`.
 //
 // Do not decay `Functor` before testing this, lest it give an incorrect result
 // for overloads with different ref-qualifiers.
 template <typename Functor, typename... BoundArgs>
 concept HasOverloadedCallOp = requires {
   // The functor must be invocable with the bound args.
   requires requires(Functor&& f, BoundArgs&&... args) {
     std::forward<Functor>(f)(std::forward<BoundArgs>(args)...);
   };
   // Now exclude invocables that are not cases of overloaded `operator()()`s:
   // * `operator()()` exists, but isn't overloaded
   requires !HasNonOverloadedCallOp<std::decay_t<Functor>>;
   // * Function pointer (doesn't have `operator()()`)
   requires !std::is_pointer_v<std::decay_t<Functor>>;
   // * Block pointer (doesn't have `operator()()`)
   requires !IsObjCArcBlockPointer<std::decay_t<Functor>>;
 };

 // `ForceVoidReturn<>` converts a signature to have a `void` return type.
 template <typename Sig>
 struct ForceVoidReturn;

 template <typename R, typename... Args>
 struct ForceVoidReturn<R(Args...)> {
   using RunType = void(Args...);
 };

 // `FunctorTraits<>`
 //
 // See description at top of file. This must be declared here so it can be
 // referenced in `DecayedFunctorTraits`.
 template <typename Functor, typename... BoundArgs>
 struct FunctorTraits;

 // Provides functor traits for pre-decayed functor types.
 template <typename Functor, typename... BoundArgs>
 struct DecayedFunctorTraits;

 // Callable types.
 // This specialization handles lambdas (captureless and capturing) and functors
 // with a call operator. Capturing lambdas and stateful functors are explicitly
 // disallowed by `BindHelper<>::Bind()`; e.g.:
 // ```
 //   // Captureless lambda: Allowed
 //   [] { return 42; };
 //
 //   // Capturing lambda: Disallowed
 //   int x;
 //   [x] { return x; };
 //
 //   // Empty class with `operator()()`: Allowed
 //   struct Foo {
 //     void operator()() const {}
 //     // No non-`static` member variables and no virtual functions.
 //   };
 // ```
 template <typename Functor, typename... BoundArgs>
   requires HasNonOverloadedCallOp<Functor>
 struct DecayedFunctorTraits<Functor, BoundArgs...> {
   using RunType = ExtractCallableRunType<Functor>;
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = false;
   static constexpr bool is_callback = false;
   static constexpr bool is_stateless = std::is_empty_v<Functor>;

   template <typename RunFunctor, typename... RunArgs>
   static ExtractReturnType<RunType> Invoke(RunFunctor&& functor,
                                            RunArgs&&... args) {
     return std::forward<RunFunctor>(functor)(std::forward<RunArgs>(args)...);
   }
 };

 // Functions.
 template <typename R, typename... Args, typename... BoundArgs>
 struct DecayedFunctorTraits<R (*)(Args...), BoundArgs...> {
   using RunType = R(Args...);
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = true;
   static constexpr bool is_callback = false;
   static constexpr bool is_stateless = true;

   template <typename Function, typename... RunArgs>
   static R Invoke(Function&& function, RunArgs&&... args) {
     return std::forward<Function>(function)(std::forward<RunArgs>(args)...);
   }
 };

 template <typename R, typename... Args, typename... BoundArgs>
 struct DecayedFunctorTraits<R (*)(Args...) noexcept, BoundArgs...>
     : DecayedFunctorTraits<R (*)(Args...), BoundArgs...> {};

 #if BUILDFLAG(IS_WIN) && !defined(ARCH_CPU_64_BITS)

 // `__stdcall` and `__fastcall` functions.
 #define BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS(conv, quals) \
   template <typename R, typename... Args, typename... BoundArgs>              \
   struct DecayedFunctorTraits<R(conv*)(Args...) quals, BoundArgs...>          \
       : DecayedFunctorTraits<R (*)(Args...) quals, BoundArgs...> {}

 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS(__stdcall, );
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS(__stdcall, noexcept);
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS(__fastcall, );
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS(__fastcall, noexcept);

 #undef BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONV_AND_QUALS
 #endif  // BUILDFLAG(IS_WIN) && !defined(ARCH_CPU_64_BITS)

 #if __OBJC__ && HAS_FEATURE(objc_arc)

 // Objective-C blocks. Blocks can be bound as the compiler will ensure their
 // lifetimes will be correctly managed.
 template <typename R, typename... Args, typename... BoundArgs>
 struct DecayedFunctorTraits<R (^)(Args...), BoundArgs...> {
   using RunType = R(Args...);
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = true;
   static constexpr bool is_callback = false;
   static constexpr bool is_stateless = true;

   template <typename BlockType, typename... RunArgs>
   static R Invoke(BlockType&& block, RunArgs&&... args) {
     // According to LLVM documentation (§ 6.3), "local variables of automatic
     // storage duration do not have precise lifetime." Use
     // `objc_precise_lifetime` to ensure that the Objective-C block is not
     // deallocated until it has finished executing even if the `Callback<>` is
     // destroyed during the block execution.
     // https://clang.llvm.org/docs/AutomaticReferenceCounting.html#precise-lifetime-semantics
     __attribute__((objc_precise_lifetime)) R (^scoped_block)(Args...) = block;
     return scoped_block(std::forward<RunArgs>(args)...);
   }
 };

 #endif  // __OBJC__ && HAS_FEATURE(objc_arc)

 // Methods.
 template <typename R,
           typename Receiver,
           typename... Args,
           typename... BoundArgs>
 struct DecayedFunctorTraits<R (Receiver::*)(Args...), BoundArgs...> {
   using RunType = R(Receiver*, Args...);
   static constexpr bool is_method = true;
   static constexpr bool is_nullable = true;
   static constexpr bool is_callback = false;
   static constexpr bool is_stateless = true;

   template <typename Method, typename ReceiverPtr, typename... RunArgs>
   static R Invoke(Method method,
                   ReceiverPtr&& receiver_ptr,
                   RunArgs&&... args) {
     return ((*receiver_ptr).*method)(std::forward<RunArgs>(args)...);
   }
 };

 template <typename R,
           typename Receiver,
           typename... Args,
           typename... BoundArgs>
 struct DecayedFunctorTraits<R (Receiver::*)(Args...) const, BoundArgs...>
     : DecayedFunctorTraits<R (Receiver::*)(Args...), BoundArgs...> {
   using RunType = R(const Receiver*, Args...);
 };

 #define BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONST_AND_QUALS(constqual, \
                                                                   quals)     \
   template <typename R, typename Receiver, typename... Args,                 \
             typename... BoundArgs>                                           \
   struct DecayedFunctorTraits<R (Receiver::*)(Args...) constqual quals,      \
                               BoundArgs...>                                  \
       : DecayedFunctorTraits<R (Receiver::*)(Args...) constqual,             \
                              BoundArgs...> {}

 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONST_AND_QUALS(, noexcept);
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONST_AND_QUALS(const, noexcept);

 #undef BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_WITH_CONST_AND_QUALS

 #if BUILDFLAG(IS_WIN) && !defined(ARCH_CPU_64_BITS)

 // `__stdcall` methods.
 #define BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS(quals)  \
   template <typename R, typename Receiver, typename... Args,            \
             typename... BoundArgs>                                      \
   struct DecayedFunctorTraits<R (__stdcall Receiver::*)(Args...) quals, \
                               BoundArgs...>                             \
       : public DecayedFunctorTraits<R (Receiver::*)(Args...) quals,     \
                                     BoundArgs...> {}

 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS();
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS(const);
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS(noexcept);
 BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS(const noexcept);

 #undef BIND_INTERNAL_DECAYED_FUNCTOR_TRAITS_STDCALL_WITH_QUALS

 #endif  // BUILDFLAG(IS_WIN) && !defined(ARCH_CPU_64_BITS)

 // `IgnoreResult`s.
 template <typename T, typename... BoundArgs>
 struct DecayedFunctorTraits<IgnoreResultHelper<T>, BoundArgs...>
     : FunctorTraits<T, BoundArgs...> {
   using RunType = typename ForceVoidReturn<
       typename FunctorTraits<T, BoundArgs...>::RunType>::RunType;

   template <typename IgnoreResultType, typename... RunArgs>
   static void Invoke(IgnoreResultType&& ignore_result_helper,
                      RunArgs&&... args) {
     FunctorTraits<T, BoundArgs...>::Invoke(
         std::forward<IgnoreResultType>(ignore_result_helper).functor_,
         std::forward<RunArgs>(args)...);
   }
 };

 // `OnceCallback`s.
 template <typename R, typename... Args, typename... BoundArgs>
 struct DecayedFunctorTraits<OnceCallback<R(Args...)>, BoundArgs...> {
   using RunType = R(Args...);
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = true;
   static constexpr bool is_callback = true;
   static constexpr bool is_stateless = true;

   template <typename CallbackType, typename... RunArgs>
   static R Invoke(CallbackType&& callback, RunArgs&&... args) {
     DCHECK(!callback.is_null());
     return std::forward<CallbackType>(callback).Run(
         std::forward<RunArgs>(args)...);
   }
 };

 // `RepeatingCallback`s.
 template <typename R, typename... Args, typename... BoundArgs>
 struct DecayedFunctorTraits<RepeatingCallback<R(Args...)>, BoundArgs...> {
   using RunType = R(Args...);
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = true;
   static constexpr bool is_callback = true;
   static constexpr bool is_stateless = true;

   template <typename CallbackType, typename... RunArgs>
   static R Invoke(CallbackType&& callback, RunArgs&&... args) {
     DCHECK(!callback.is_null());
     return std::forward<CallbackType>(callback).Run(
         std::forward<RunArgs>(args)...);
   }
 };

 // For most functors, the traits should not depend on how the functor is passed,
 // so decay the functor.
 template <typename Functor, typename... BoundArgs>
 // This requirement avoids "implicit instantiation of undefined template" errors
 // when the underlying `DecayedFunctorTraits<>` cannot be instantiated. Instead,
 // this template will also not be instantiated, and the caller can detect and
 // handle that.
   requires IsComplete<DecayedFunctorTraits<std::decay_t<Functor>, BoundArgs...>>
 struct FunctorTraits<Functor, BoundArgs...>
     : DecayedFunctorTraits<std::decay_t<Functor>, BoundArgs...> {};

 // For `overloaded operator()()`s, it's possible the ref qualifiers of the
 // functor matter, so be careful to use the undecayed type.
 template <typename Functor, typename... BoundArgs>
   requires HasOverloadedCallOp<Functor, BoundArgs...>
 struct FunctorTraits<Functor, BoundArgs...> {
   // For an overloaded operator()(), it is not possible to resolve the
   // actual declared type. Since it is invocable with the bound args, make up a
   // signature based on their types.
   using RunType = decltype(std::declval<Functor>()(
       std::declval<BoundArgs>()...))(std::decay_t<BoundArgs>...);
   static constexpr bool is_method = false;
   static constexpr bool is_nullable = false;
   static constexpr bool is_callback = false;
   static constexpr bool is_stateless = std::is_empty_v<std::decay_t<Functor>>;

   template <typename RunFunctor, typename... RunArgs>
   static ExtractReturnType<RunType> Invoke(RunFunctor&& functor,
                                            RunArgs&&... args) {
     return std::forward<RunFunctor>(functor)(std::forward<RunArgs>(args)...);
   }
 };

 // `StorageTraits<>`
 //
 // See description at top of file.
 template <typename T>
 struct StorageTraits {
   // The type to use for storing the bound arg inside `BindState`.
   using Type = T;

   // True iff all compile-time preconditions for using this specialization are
   // satisfied. Specializations that set this to `false` should ensure a
   // `static_assert()` explains why.
   static constexpr bool value = true;
 };

 // For `T*`, store as `UnretainedWrapper<T>` for safety, as it internally uses
 // `raw_ptr<T>` (when possible).
 template <typename T>
 struct StorageTraits<T*> {
   using Type = UnretainedWrapper<T, unretained_traits::MayNotDangle>;
   static constexpr bool value = Type::value;
 };

 // For `raw_ptr<T>`, store as `UnretainedWrapper<T>` for safety. This may seem
 // contradictory, but this ensures guaranteed protection for the pointer even
 // during execution of callbacks with parameters of type `raw_ptr<T>`.
 template <typename T, RawPtrTraits PtrTraits>
 struct StorageTraits<raw_ptr<T, PtrTraits>> {
   using Type = UnretainedWrapper<T, unretained_traits::MayNotDangle, PtrTraits>;
   static constexpr bool value = Type::value;
 };

 // Unwrap `std::reference_wrapper` and store it in a custom wrapper so that
 // references are also protected with `raw_ptr<T>`.
 template <typename T>
 struct StorageTraits<std::reference_wrapper<T>> {
   using Type = UnretainedRefWrapper<T, unretained_traits::MayNotDangle>;
   static constexpr bool value = Type::value;
 };

 template <typename T>
 using ValidateStorageTraits = StorageTraits<std::decay_t<T>>;

 // `InvokeHelper<>`
 //
 // There are 2 logical `InvokeHelper<>` specializations: normal, weak.
 //
 // The normal type just calls the underlying runnable.
 //
 // Weak calls need special syntax that is applied to the first argument to check
 // if they should no-op themselves.
 template <bool is_weak_call,
           typename Traits,
           typename ReturnType,
           size_t... indices>
 struct InvokeHelper;

 template <typename Traits, typename ReturnType, size_t... indices>
 struct InvokeHelper<false, Traits, ReturnType, indices...> {
   template <typename Functor, typename BoundArgsTuple, typename... RunArgs>
   static inline ReturnType MakeItSo(Functor&& functor,
                                     BoundArgsTuple&& bound,
                                     RunArgs&&... args) {
     return Traits::Invoke(
         Unwrap(std::forward<Functor>(functor)),
         Unwrap(std::get<indices>(std::forward<BoundArgsTuple>(bound)))...,
         std::forward<RunArgs>(args)...);
   }
 };

 template <typename Traits,
           typename ReturnType,
           size_t index_target,
           size_t... index_tail>
 struct InvokeHelper<true, Traits, ReturnType, index_target, index_tail...> {
   template <typename Functor, typename BoundArgsTuple, typename... RunArgs>
   static inline void MakeItSo(Functor&& functor,
                               BoundArgsTuple&& bound,
                               RunArgs&&... args) {
     static_assert(index_target == 0);
     // Note the validity of the weak pointer should be tested _after_ it is
     // unwrapped, otherwise it creates a race for weak pointer implementations
     // that allow cross-thread usage and perform `Lock()` in `Unwrap()` traits.
     const auto& target = Unwrap(std::get<0>(bound));
     if (!target) {
       return;
     }
     Traits::Invoke(
         Unwrap(std::forward<Functor>(functor)), target,
         Unwrap(std::get<index_tail>(std::forward<BoundArgsTuple>(bound)))...,
         std::forward<RunArgs>(args)...);
   }
 };

 // `Invoker<>`
 //
 // See description at the top of the file.
 template <typename Traits, typename StorageType, typename UnboundRunType>
 struct Invoker;

 template <typename Traits,
           typename StorageType,
           typename R,
           typename... UnboundArgs>
 struct Invoker<Traits, StorageType, R(UnboundArgs...)> {
  private:
   using Indices = std::make_index_sequence<
       std::tuple_size_v<decltype(StorageType::bound_args_)>>;

  public:
   static R RunOnce(BindStateBase* base,
                    PassingType<UnboundArgs>... unbound_args) {
     auto* const storage = static_cast<StorageType*>(base);
     return RunImpl(std::move(storage->functor_),
                    std::move(storage->bound_args_), Indices(),
                    std::forward<UnboundArgs>(unbound_args)...);
   }

   static R Run(BindStateBase* base, PassingType<UnboundArgs>... unbound_args) {
     auto* const storage = static_cast<const StorageType*>(base);
     return RunImpl(storage->functor_, storage->bound_args_, Indices(),
                    std::forward<UnboundArgs>(unbound_args)...);
   }

  private:
   // The "templated struct with a lambda that asserts" pattern below is used
   // repeatedly in Bind/Callback code to verify compile-time preconditions. The
   // goal is to print only the root cause failure when users violate a
   // precondition, and not also a host of resulting compile errors.
   //
   // There are three key aspects:
   //   1. By placing the assertion inside a lambda that initializes a variable,
   //      the assertion will not be verified until the compiler tries to read
   //      the value of that variable. This allows the containing types to be
   //      complete. As a result, code that needs to know if the assertion failed
   //      can read the variable's value and get the right answer. (If we instead
   //      placed the assertion at struct scope, the resulting type would be
   //      incomplete when the assertion failed; in practice, reading a
   //      `constexpr` member of an incomplete type seems to return the default
   //      value regardless of what the code tried to set the value to, which
   //      makes it impossible for other code to check whether the assertion
   //      failed.)
   //   2. Code that will not successfully compile unless the assertion holds is
   //      guarded by a constexpr if that checks the variable.
   //   3. By placing the variable inside an independent, templated struct and
   //      naming it `value`, we allow checking multiple conditions via
   //      `std::conjunction_v<>`. This short-circuits type instantiation, so
   //      that when one condition fails, the others are never examined and thus
   //      never assert. As a result, we can verify dependent conditions without
   //      worrying that "if one fails, we'll get errors from several others".
   //      (This would not be true if we simply checked all the values with `&&`,
   //      which would instantiate all the types before evaluating the
   //      expression.)
   //
   // For caller convenience and to avoid potential repetition, the actual
   // condition to be checked is always used as the default value of a template
   // argument, so callers can simply instantiate the struct with no template
   // params to verify the condition.

   // Weak calls are only supported for functions with a `void` return type.
   // Otherwise, the desired function result would be unclear if the `WeakPtr<>`
   // is invalidated. In theory, we could support default-constructible return
   // types (and return the default value) or allow callers to specify a default
   // return value via a template arg. It's not clear these are necessary.
   template <bool is_weak_call, bool v = !is_weak_call || std::is_void_v<R>>
   struct WeakCallReturnsVoid {
     static constexpr bool value = [] {
       static_assert(v,
                     "WeakPtrs can only bind to methods without return values.");
       return v;
     }();
   };

   template <typename Functor, typename BoundArgsTuple, size_t... indices>
   static inline R RunImpl(Functor&& functor,
                           BoundArgsTuple&& bound,
                           std::index_sequence<indices...>,
                           UnboundArgs&&... unbound_args) {
 #if PA_BUILDFLAG(USE_ASAN_BACKUP_REF_PTR)
     RawPtrAsanBoundArgTracker raw_ptr_asan_bound_arg_tracker;
     raw_ptr_asan_bound_arg_tracker.AddArgs(
         std::get<indices>(std::forward<BoundArgsTuple>(bound))...,
         std::forward<UnboundArgs>(unbound_args)...);
 #endif  // PA_BUILDFLAG(USE_ASAN_BACKUP_REF_PTR)

     using DecayedArgsTuple = std::decay_t<BoundArgsTuple>;
     static constexpr bool kIsWeakCall =
         kIsWeakMethod<Traits::is_method,
                       std::tuple_element_t<indices, DecayedArgsTuple>...>;
     if constexpr (WeakCallReturnsVoid<kIsWeakCall>::value) {
       // Do not `Unwrap()` here, as that immediately triggers dangling pointer
       // detection. Dangling pointer detection should only be triggered if the
       // callback is not cancelled, but cancellation status is not determined
       // until later inside the `InvokeHelper::MakeItSo()` specialization for
       // weak calls.
       //
       // Dangling pointers when invoking a cancelled callback are not considered
       // a memory safety error because protecting raw pointers usage with weak
       // receivers (where the weak receiver usually own the pointed objects) is
       // a common and broadly used pattern in the codebase.
       return InvokeHelper<kIsWeakCall, Traits, R, indices...>::MakeItSo(
           std::forward<Functor>(functor), std::forward<BoundArgsTuple>(bound),
           std::forward<UnboundArgs>(unbound_args)...);
     }
   }
 };

 // Allow binding a method call with no receiver.
 // TODO(crbug.com/41484339): Remove or make safe.
 template <typename... Unused>
 void VerifyMethodReceiver(Unused&&...) {}

 template <typename Receiver, typename... Unused>
 void VerifyMethodReceiver(Receiver&& receiver, Unused&&...) {
   // Asserts that a callback is not the first owner of a ref-counted receiver.
   if constexpr (IsPointerOrRawPtr<std::decay_t<Receiver>> &&
                 IsRefCountedType<RemovePointerT<std::decay_t<Receiver>>>) {
     DCHECK(receiver);

     // It's error prone to make the implicit first reference to ref-counted
     // types. In the example below, `BindOnce()` would make the implicit first
     // reference to the ref-counted `Foo`. If `PostTask()` failed or the posted
     // task ran fast enough, the newly created instance could be destroyed
     // before `oo` makes another reference.
     // ```
     //   Foo::Foo() {
     //     ThreadPool::PostTask(FROM_HERE, BindOnce(&Foo::Bar, this));
     //   }
     //
     //   scoped_refptr<Foo> oo = new Foo();
     // ```
     //
     // Hence, `Bind()` refuses to create the first reference to ref-counted
     // objects, and `DCHECK()`s otherwise. As above, that typically happens
     // around `PostTask()` in their constructors, and such objects can be
     // destroyed before `new` returns if the tasks resolve fast enough.
     //
     // Instead, consider adding a static factory, and keeping the first
     // reference alive explicitly.
     // ```
     //   // static
     //   scoped_refptr<Foo> Foo::Create() {
     //     auto foo = base::WrapRefCounted(new Foo());
     //     ThreadPool::PostTask(FROM_HERE, BindOnce(&Foo::Bar, foo));
     //     return foo;
     //   }
     //
     //   scoped_refptr<Foo> oo = Foo::Create();
     // ```
     DCHECK(receiver->HasAtLeastOneRef());
   }
 }

 // `BindState<>`
 //
 // This stores all the state passed into `Bind()`.
 template <bool is_method,
           bool is_nullable,
           bool is_callback,
           typename Functor,
           typename... BoundArgs>
 struct BindState final : BindStateBase {
  private:
   using BoundArgsTuple = std::tuple<BoundArgs...>;

  public:
   template <typename ForwardFunctor, typename... ForwardBoundArgs>
   static BindState* Create(BindStateBase::InvokeFuncStorage invoke_func,
                            ForwardFunctor&& functor,
                            ForwardBoundArgs&&... bound_args) {
     if constexpr (is_method) {
       VerifyMethodReceiver(bound_args...);
     }
     return new BindState(invoke_func, std::forward<ForwardFunctor>(functor),
                          std::forward<ForwardBoundArgs>(bound_args)...);
   }

   Functor functor_;
   BoundArgsTuple bound_args_;

  private:
   using CancellationTraits =
       CallbackCancellationTraits<Functor, BoundArgsTuple>;

   template <typename ForwardFunctor, typename... ForwardBoundArgs>
     requires CancellationTraits::is_cancellable
   BindState(BindStateBase::InvokeFuncStorage invoke_func,
             ForwardFunctor&& functor,
             ForwardBoundArgs&&... bound_args)
       : BindStateBase(invoke_func, &Destroy, &QueryCancellationTraits),
         functor_(std::forward<ForwardFunctor>(functor)),
         bound_args_(std::forward<ForwardBoundArgs>(bound_args)...) {
     CheckFunctorIsNonNull();
   }

   template <typename ForwardFunctor, typename... ForwardBoundArgs>
     requires(!CancellationTraits::is_cancellable)
   BindState(BindStateBase::InvokeFuncStorage invoke_func,
             ForwardFunctor&& functor,
             ForwardBoundArgs&&... bound_args)
       : BindStateBase(invoke_func, &Destroy),
         functor_(std::forward<ForwardFunctor>(functor)),
         bound_args_(std::forward<ForwardBoundArgs>(bound_args)...) {
     CheckFunctorIsNonNull();
   }

   ~BindState() = default;

   static bool QueryCancellationTraits(
       const BindStateBase* base,
       BindStateBase::CancellationQueryMode mode) {
     auto* const storage = static_cast<const BindState*>(base);
     static constexpr std::make_index_sequence<sizeof...(BoundArgs)> kIndices;
     return (mode == BindStateBase::CancellationQueryMode::kIsCancelled)
                ? storage->IsCancelled(kIndices)
                : storage->MaybeValid(kIndices);
   }

   static void Destroy(const BindStateBase* self) {
     delete static_cast<const BindState*>(self);
   }

   // Helpers to do arg tuple expansion.
   template <size_t... indices>
   bool IsCancelled(std::index_sequence<indices...>) const {
     return CancellationTraits::IsCancelled(functor_,
                                            std::get<indices>(bound_args_)...);
   }

   template <size_t... indices>
   bool MaybeValid(std::index_sequence<indices...>) const {
     return CancellationTraits::MaybeValid(functor_,
                                           std::get<indices>(bound_args_)...);
   }

   void CheckFunctorIsNonNull() const {
     if constexpr (is_nullable) {
       // Check the validity of `functor_` to avoid hard-to-diagnose crashes.
       // Ideally we'd do this unconditionally, but release builds limit this to
       // the case of nested callbacks (e.g. `Bind(callback, ...)`) to limit
       // binary size impact.
       if constexpr (is_callback) {
         CHECK(!!functor_);
       } else {
         DCHECK(!!functor_);
       }
     }
   }
 };

 template <typename... BoundArgs>
 struct ValidateBindStateTypeCommonChecks {
  private:
   // Refcounted parameters must be passed as `scoped_refptr` instead of raw
   // pointers, to ensure they are not deleted before use.
   // TODO(danakj): Ban native references and `std::reference_wrapper` too.
   template <typename T,
             bool v =
                 (IsRawRef<T> && IsRefCountedType<base::RemoveRawRefT<T>>) ||
                 (IsPointerOrRawPtr<T> &&
                  IsRefCountedType<base::RemovePointerT<T>>)>
   struct RefCountedTypeNotPassedByRawPointer {
     static constexpr bool value = [] {
       static_assert(
           !v, "A parameter is a refcounted type and needs scoped_refptr.");
       return !v;
     }();
   };

  public:
   using CommonCheckResult = std::conjunction<
       RefCountedTypeNotPassedByRawPointer<std::decay_t<BoundArgs>>...,
       ValidateStorageTraits<BoundArgs>...>;
 };

 // Used to determine and validate the appropriate `BindState`. The
 // specializations below cover all cases. The members are similar in intent to
 // those in `StorageTraits`; see comments there.
 template <bool is_method,
           bool is_nullable,
           bool is_callback,
           typename Functor,
           typename... BoundArgs>
 struct ValidateBindStateType;

 template <bool is_nullable,
           bool is_callback,
           typename Functor,
           typename... BoundArgs>
 struct ValidateBindStateType<false,
                              is_nullable,
                              is_callback,
                              Functor,
                              BoundArgs...> {
   using Type = BindState<false,
                          is_nullable,
                          is_callback,
                          std::decay_t<Functor>,
                          typename ValidateStorageTraits<BoundArgs>::Type...>;
   static constexpr bool value =
       ValidateBindStateTypeCommonChecks<BoundArgs...>::CommonCheckResult::value;
 };

 template <bool is_nullable, bool is_callback, typename Functor>
 struct ValidateBindStateType<true, is_nullable, is_callback, Functor> {
   using Type = BindState<true, is_nullable, is_callback, std::decay_t<Functor>>;
   static constexpr bool value = true;
 };

 template <bool is_nullable,
           bool is_callback,
           typename Functor,
           typename Receiver,
           typename... BoundArgs>
 struct ValidateBindStateType<true,
                              is_nullable,
                              is_callback,
                              Functor,
                              Receiver,
                              BoundArgs...> {
  private:
   using DecayedReceiver = std::decay_t<Receiver>;
   using ReceiverStorageType =
       typename MethodReceiverStorage<DecayedReceiver>::Type;

   template <bool v = !std::is_array_v<std::remove_reference_t<Receiver>>>
   struct FirstBoundArgIsNotArray {
     static constexpr bool value = [] {
       static_assert(v, "First bound argument to a method cannot be an array.");
       return v;
     }();
   };

   template <bool v = !IsRawRef<DecayedReceiver>>
   struct ReceiverIsNotRawRef {
     static constexpr bool value = [] {
       static_assert(v, "Receivers may not be raw_ref<T>. If using a raw_ref<T> "
                        "here is safe and has no lifetime concerns, use "
                        "base::Unretained() and document why it's safe.");
       return v;
     }();
   };

   template <bool v = !IsPointerOrRawPtr<DecayedReceiver> ||
                      IsRefCountedType<RemovePointerT<DecayedReceiver>>>
   struct ReceiverIsNotRawPtr {
     static constexpr bool value = [] {
       static_assert(v,
                     "Receivers may not be raw pointers. If using a raw pointer "
                     "here is safe and has no lifetime concerns, use "
                     "base::Unretained() and document why it's safe.");
       return v;
     }();
   };

  public:
   using Type = BindState<true,
                          is_nullable,
                          is_callback,
                          std::decay_t<Functor>,
                          ReceiverStorageType,
                          typename ValidateStorageTraits<BoundArgs>::Type...>;
   static constexpr bool value =
       std::conjunction_v<FirstBoundArgIsNotArray<>,
                          ReceiverIsNotRawRef<>,
                          ReceiverIsNotRawPtr<>,
                          typename ValidateBindStateTypeCommonChecks<
                              BoundArgs...>::CommonCheckResult>;
 };

 // Transforms `T` into an unwrapped type, which is passed to the target
 // function; e.g.:
 // * `is_once` cases:
 // ** `TransformToUnwrappedType<true, int&&>` -> `int&&`
 // ** `TransformToUnwrappedType<true, const int&>` -> `int&&`
 // ** `TransformToUnwrappedType<true, OwnedWrapper<int>&>` -> `int*&&`
 // * `!is_once` cases:
 // ** `TransformToUnwrappedType<false, int&&>` -> `const int&`
 // ** `TransformToUnwrappedType<false, const int&>` -> `const int&`
 // ** `TransformToUnwrappedType<false, OwnedWrapper<int>&>` -> `int* const &`
 template <bool is_once,
           typename T,
           typename StoredType = std::decay_t<T>,
           typename ForwardedType =
               std::conditional_t<is_once, StoredType&&, const StoredType&>>
 using TransformToUnwrappedType =
     decltype(Unwrap(std::declval<ForwardedType>()));

 // Used to convert `this` arguments to underlying pointer types; e.g.:
 //   `int*` -> `int*`
 //   `std::unique_ptr<int>` -> `int*`
 //   `int` -> (assertion failure; `this` must be a pointer-like object)
 template <typename T>
 struct ValidateReceiverType {
  private:
   // Pointer-like receivers use a different specialization, so this never
   // succeeds.
   template <bool v = false>
   struct ReceiverMustBePointerLike {
     static constexpr bool value = [] {
       static_assert(v,
                     "Cannot convert `this` argument to address. Method calls "
                     "must be bound using a pointer-like `this` argument.");
       return v;
     }();
   };

  public:
   // These members are similar in intent to those in `StorageTraits`; see
   // comments there.
   using Type = T;
   static constexpr bool value = ReceiverMustBePointerLike<>::value;
 };

 template <typename T>
   requires requires(T&& t) { base::to_address(t); }
 struct ValidateReceiverType<T> {
   using Type = decltype(base::to_address(std::declval<T>()));
   static constexpr bool value = true;
 };

 // Transforms `Args` into unwrapped types, and packs them into a `TypeList`. If
 // `is_method` is true, tries to dereference the first argument to support smart
 // pointers.
 template <bool is_once, bool is_method, typename... Args>
 struct ValidateUnwrappedTypeList {
   // These members are similar in intent to those in `StorageTraits`; see
   // comments there.
   using Type = TypeList<TransformToUnwrappedType<is_once, Args>...>;
   static constexpr bool value = true;
 };

 template <bool is_once, typename Receiver, typename... Args>
 struct ValidateUnwrappedTypeList<is_once, true, Receiver, Args...> {
  private:
   using ReceiverStorageType =
       typename MethodReceiverStorage<std::decay_t<Receiver>>::Type;
   using UnwrappedReceiver =
       TransformToUnwrappedType<is_once, ReceiverStorageType>;
   using ValidatedReceiver = ValidateReceiverType<UnwrappedReceiver>;

  public:
   using Type = TypeList<typename ValidatedReceiver::Type,
                         TransformToUnwrappedType<is_once, Args>...>;
   static constexpr bool value = ValidatedReceiver::value;
 };

 // `IsUnretainedMayDangle` is true iff `StorageType` is marked with
 // `unretained_traits::MayDangle`. Note that it is false for
 // `unretained_traits::MayDangleUntriaged`.
 template <typename StorageType>
 inline constexpr bool IsUnretainedMayDangle = false;

 template <typename T, RawPtrTraits PtrTraits>
 inline constexpr bool IsUnretainedMayDangle<
     UnretainedWrapper<T, unretained_traits::MayDangle, PtrTraits>> = true;

 // `UnretainedAndRawPtrHaveCompatibleTraits` is true iff `StorageType` is marked
 // with `unretained_traits::MayDangle`, `FunctionParamType` is a `raw_ptr`, and
 // `StorageType::GetPtrType` is the same type as `FunctionParamType`.
 template <typename StorageType, typename FunctionParamType>
 inline constexpr bool UnretainedAndRawPtrHaveCompatibleTraits = false;

 template <typename T,
           RawPtrTraits PtrTraitsInUnretained,
           RawPtrTraits PtrTraitsInReceiver>
 inline constexpr bool UnretainedAndRawPtrHaveCompatibleTraits<
     UnretainedWrapper<T, unretained_traits::MayDangle, PtrTraitsInUnretained>,
     raw_ptr<T, PtrTraitsInReceiver>> =
     std::same_as<typename UnretainedWrapper<T,
                                             unretained_traits::MayDangle,
                                             PtrTraitsInUnretained>::GetPtrType,
                  raw_ptr<T, PtrTraitsInReceiver>>;

 // Helpers to make error messages slightly more readable.
 template <int i>
 struct BindArgument {
   template <typename ForwardingType>
   struct ForwardedAs {
     template <typename FunctorParamType>
     struct ToParamWithType {
       static constexpr bool kRawPtr = IsRawPtr<FunctorParamType>;
       static constexpr bool kRawPtrMayBeDangling =
           IsRawPtrMayDangle<FunctorParamType>;
       static constexpr bool kCanBeForwardedToBoundFunctor =
           std::is_convertible_v<ForwardingType, FunctorParamType>;

       // If the bound type can't be forwarded, then test if `FunctorParamType`
       // is a non-const lvalue reference and a reference to the unwrapped type
       // could have been successfully forwarded.
       static constexpr bool kIsUnwrappedForwardableNonConstReference =
           std::is_lvalue_reference_v<FunctorParamType> &&
           !std::is_const_v<std::remove_reference_t<FunctorParamType>> &&
           std::is_convertible_v<std::decay_t<ForwardingType>&,
                                 FunctorParamType>;

       // Also intentionally drop the `const` qualifier from `ForwardingType`, to
       // test if it could have been successfully forwarded if `Passed()` had
       // been used.
       static constexpr bool kWouldBeForwardableWithPassed =
           std::is_convertible_v<std::decay_t<ForwardingType>&&,
                                 FunctorParamType>;
     };
   };

   template <typename BoundAsType>
   struct BoundAs {
     template <typename StorageType>
     struct StoredAs {
       static constexpr bool kBindArgumentCanBeCaptured =
           std::constructible_from<StorageType, BoundAsType>;

       // If the argument can't be captured, intentionally drop the `const`
       // qualifier from `BoundAsType`, to test if it could have been
       // successfully captured if `std::move()` had been used.
       static constexpr bool kWouldBeCapturableWithStdMove =
           std::constructible_from<StorageType, std::decay_t<BoundAsType>&&>;
     };
   };

   template <typename FunctionParamType>
   struct ToParamWithType {
     template <typename StorageType>
     struct StoredAs {
       static constexpr bool kBoundPtrMayDangle =
           IsUnretainedMayDangle<StorageType>;

       static constexpr bool kMayDangleAndMayBeDanglingHaveMatchingTraits =
           UnretainedAndRawPtrHaveCompatibleTraits<StorageType,
                                                   FunctionParamType>;
     };
   };
 };

 // Helper to assert that parameter `i` of type `Arg` can be bound, which means:
 // * `Arg` can be retained internally as `Storage`
 // * `Arg` can be forwarded as `Unwrapped` to `Param`
 template <int i,
           bool is_method,
           typename Arg,
           typename Storage,
           typename Unwrapped,
           typename Param>
 struct ParamCanBeBound {
  private:
   using UnwrappedParam = BindArgument<i>::template ForwardedAs<
       Unwrapped>::template ToParamWithType<Param>;
   using ParamStorage = BindArgument<i>::template ToParamWithType<
       Param>::template StoredAs<Storage>;
   using BoundStorage =
       BindArgument<i>::template BoundAs<Arg>::template StoredAs<Storage>;

   template <bool v = !UnwrappedParam::kRawPtr ||
                      UnwrappedParam::kRawPtrMayBeDangling>
   struct NotRawPtr {
     static constexpr bool value = [] {
       static_assert(
           v, "Use T* or T& instead of raw_ptr<T> for function parameters, "
              "unless you must mark the parameter as MayBeDangling<T>.");
       return v;
     }();
   };

   template <bool v = !ParamStorage::kBoundPtrMayDangle ||
                      UnwrappedParam::kRawPtrMayBeDangling ||
                      // Exempt `this` pointer as it is not passed as a regular
                      // function argument.
                      (is_method && i == 0)>
   struct MayBeDanglingPtrPassedCorrectly {
     static constexpr bool value = [] {
       static_assert(v, "base::UnsafeDangling() pointers should only be passed "
                        "to parameters marked MayBeDangling<T>.");
       return v;
     }();
   };

   template <bool v =
                 !UnwrappedParam::kRawPtrMayBeDangling ||
                 (ParamStorage::kBoundPtrMayDangle &&
                  ParamStorage::kMayDangleAndMayBeDanglingHaveMatchingTraits)>
   struct MayDangleAndMayBeDanglingHaveMatchingTraits {
     static constexpr bool value = [] {
       static_assert(
           v, "Pointers passed to MayBeDangling<T> parameters must be created "
              "by base::UnsafeDangling() with the same RawPtrTraits.");
       return v;
     }();
   };

   // With `BindRepeating()`, there are two decision points for how to handle a
   // move-only type:
   //
   // 1. Whether the move-only argument should be moved into the internal
   //    `BindState`. Either `std::move()` or `Passed()` is sufficient to trigger
   //    move-only semantics.
   // 2. Whether or not the bound, move-only argument should be moved to the
   //    bound functor when invoked. When the argument is bound with `Passed()`,
   //    invoking the callback will destructively move the bound, move-only
   //    argument to the bound functor. In contrast, if the argument is bound
   //    with `std::move()`, `RepeatingCallback` will attempt to call the bound
   //    functor with a constant reference to the bound, move-only argument. This
   //    will fail if the bound functor accepts that argument by value, since the
   //    argument cannot be copied. It is this latter case that this
   //    assertion aims to catch.
   //
   // In contrast, `BindOnce()` only has one decision point. Once a move-only
   // type is captured by value into the internal `BindState`, the bound,
   // move-only argument will always be moved to the functor when invoked.
   // Failure to use `std::move()` will simply fail the
   // `MoveOnlyTypeMustUseStdMove` assertion below instead.
   //
   // Note: `Passed()` is a legacy of supporting move-only types when repeating
   // callbacks were the only callback type. A `RepeatingCallback` with a
   // `Passed()` argument is really a `OnceCallback` and should eventually be
   // migrated.
   template <bool v = UnwrappedParam::kCanBeForwardedToBoundFunctor ||
                      !UnwrappedParam::kWouldBeForwardableWithPassed>
   struct MoveOnlyTypeMustUseBasePassed {
     static constexpr bool value = [] {
       static_assert(v,
                     "base::BindRepeating() argument is a move-only type. Use "
                     "base::Passed() instead of std::move() to transfer "
                     "ownership from the callback to the bound functor.");
       return v;
     }();
   };

   template <bool v = UnwrappedParam::kCanBeForwardedToBoundFunctor ||
                      !UnwrappedParam::kIsUnwrappedForwardableNonConstReference>
   struct NonConstRefParamMustBeWrapped {
     static constexpr bool value = [] {
       static_assert(v,
                     "Bound argument for non-const reference parameter must be "
                     "wrapped in std::ref() or base::OwnedRef().");
       return v;
     }();
   };

   // Generic failed-to-forward message for cases that didn't match one of the
   // two assertions above.
   template <bool v = UnwrappedParam::kCanBeForwardedToBoundFunctor>
   struct CanBeForwardedToBoundFunctor {
     static constexpr bool value = [] {
       static_assert(v,
                     "Type mismatch between bound argument and bound functor's "
                     "parameter.");
       return v;
     }();
   };

   // The most common reason for failing to capture a parameter is attempting to
   // pass a move-only type as an lvalue.
   template <bool v = BoundStorage::kBindArgumentCanBeCaptured ||
                      !BoundStorage::kWouldBeCapturableWithStdMove>
   struct MoveOnlyTypeMustUseStdMove {
     static constexpr bool value = [] {
       static_assert(v,
                     "Attempting to bind a move-only type. Use std::move() to "
                     "transfer ownership to the created callback.");
       return v;
     }();
   };

   // Any other reason the parameter could not be captured.
   template <bool v = BoundStorage::kBindArgumentCanBeCaptured>
   struct BindArgumentCanBeCaptured {
     static constexpr bool value = [] {
       // In practice, failing this precondition should be rare, as the storage
       // type is deduced from the arguments passed to `Bind()`.
       static_assert(
           v, "Cannot capture argument: is the argument copyable or movable?");
       return v;
     }();
   };

  public:
   static constexpr bool value =
       std::conjunction_v<NotRawPtr<>,
                          MayBeDanglingPtrPassedCorrectly<>,
                          MayDangleAndMayBeDanglingHaveMatchingTraits<>,
                          MoveOnlyTypeMustUseBasePassed<>,
                          NonConstRefParamMustBeWrapped<>,
                          CanBeForwardedToBoundFunctor<>,
                          MoveOnlyTypeMustUseStdMove<>,
                          BindArgumentCanBeCaptured<>>;
 };

 // Takes three same-length `TypeList`s, and checks `ParamCanBeBound` for each
 // triple.
 template <bool is_method,
           typename Index,
           typename Args,
           typename UnwrappedTypeList,
           typename ParamsList>
 struct ParamsCanBeBound {
   static constexpr bool value = false;
 };

 template <bool is_method,
           size_t... Ns,
           typename... Args,
           typename... UnwrappedTypes,
           typename... Params>
 struct ParamsCanBeBound<is_method,
                         std::index_sequence<Ns...>,
                         TypeList<Args...>,
                         TypeList<UnwrappedTypes...>,
                         TypeList<Params...>> {
   static constexpr bool value =
       std::conjunction_v<ParamCanBeBound<Ns,
                                          is_method,
                                          Args,
                                          std::decay_t<Args>,
                                          UnwrappedTypes,
                                          Params>...>;
 };

 // Core implementation of `Bind()`, which checks common preconditions before
 // returning an appropriate callback.
 template <template <typename> class CallbackT>
 struct BindHelper {
  private:
   static constexpr bool kIsOnce =
       is_instantiation<CallbackT<void()>, OnceCallback>;

   template <typename Traits, bool v = IsComplete<Traits>>
   struct TraitsAreInstantiable {
     static constexpr bool value = [] {
       static_assert(
           v, "Could not determine how to invoke functor. If this functor has "
              "an overloaded operator()(), bind all arguments to it, and ensure "
              "the result will select a unique overload.");
       return v;
     }();
   };

   template <typename Functor,
             bool v = !is_instantiation<std::decay_t<Functor>, OnceCallback> ||
                      (kIsOnce && std::is_rvalue_reference_v<Functor&&> &&
                       !std::is_const_v<std::remove_reference_t<Functor>>)>
   struct OnceCallbackFunctorIsValid {
     static constexpr bool value = [] {
       if constexpr (kIsOnce) {
         static_assert(v,
                       "BindOnce() requires non-const rvalue for OnceCallback "
                       "binding, i.e. base::BindOnce(std::move(callback)).");
       } else {
         static_assert(v, "BindRepeating() cannot bind OnceCallback. Use "
                          "BindOnce() with std::move().");
       }
       return v;
     }();
   };

   template <typename... Args>
   struct NoBindArgToOnceCallbackIsBasePassed {
     static constexpr bool value = [] {
       // Can't use a defaulted template param since it can't come after `Args`.
       constexpr bool v =
           !kIsOnce ||
           (... && !is_instantiation<std::decay_t<Args>, PassedWrapper>);
       static_assert(
           v,
           "Use std::move() instead of base::Passed() with base::BindOnce().");
       return v;
     }();
   };

   template <
       typename Functor,
       bool v =
           !is_instantiation<std::remove_cvref_t<Functor>, FunctionRef> &&
           !is_instantiation<std::remove_cvref_t<Functor>, absl::FunctionRef>>
   struct NotFunctionRef {
     static constexpr bool value = [] {
       static_assert(
           v,
           "Functor may not be a FunctionRef, since that is a non-owning "
           "reference that may go out of scope before the callback executes.");
       return v;
     }();
   };

   template <typename Traits, bool v = Traits::is_stateless>
   struct IsStateless {
     static constexpr bool value = [] {
       static_assert(
           v, "Capturing lambdas and stateful functors are intentionally not "
              "supported. Use a non-capturing lambda or stateless functor (i.e. "
              "has no non-static data members) and bind arguments directly.");
       return v;
     }();
   };

   template <typename Functor, typename... Args>
   static auto BindImpl(Functor&& functor, Args&&... args) {
     // There are a lot of variables and type aliases here. An example will be
     // illustrative. Assume we call:
     // ```
     //   struct S {
     //     double f(int, const std::string&);
     //   } s;
     //   int16_t i;
     //   BindOnce(&S::f, Unretained(&s), i);
     // ```
     // This means our template params are:
     // ```
     //   template <typename> class CallbackT = OnceCallback
     //   typename Functor = double (S::*)(int, const std::string&)
     //   typename... Args =
     //       UnretainedWrapper<S, unretained_traits::MayNotDangle>, int16_t
     // ```
     // And the implementation below is effectively:
     // ```
     //   using Traits = struct {
     //     using RunType = double(S*, int, const std::string&);
     //     static constexpr bool is_method = true;
     //     static constexpr bool is_nullable = true;
     //     static constexpr bool is_callback = false;
     //     static constexpr bool is_stateless = true;
     //     ...
     //   };
     //   using ValidatedUnwrappedTypes = struct {
     //     using Type = TypeList<S*, int16_t>;
     //     static constexpr bool value = true;
     //   };
     //   using BoundArgsList = TypeList<S*, int16_t>;
     //   using RunParamsList = TypeList<S*, int, const std::string&>;
     //   using BoundParamsList = TypeList<S*, int>;
     //   using ValidatedBindState = struct {
     //     using Type =
     //         BindState<double (S::*)(int, const std::string&),
     //                   UnretainedWrapper<S, unretained_traits::MayNotDangle>,
     //                   int16_t>;
     //     static constexpr bool value = true;
     //   };
     //   if constexpr (true) {
     //     using UnboundRunType = double(const std::string&);
     //     using CallbackType = OnceCallback<double(const std::string&)>;
     //     ...
     // ```
     using Traits = FunctorTraits<TransformToUnwrappedType<kIsOnce, Functor&&>,
                                  TransformToUnwrappedType<kIsOnce, Args&&>...>;
     if constexpr (TraitsAreInstantiable<Traits>::value) {
       using ValidatedUnwrappedTypes =
           ValidateUnwrappedTypeList<kIsOnce, Traits::is_method, Args&&...>;
       using BoundArgsList = TypeList<Args...>;
       using RunParamsList = ExtractArgs<typename Traits::RunType>;
       using BoundParamsList = TakeTypeListItem<sizeof...(Args), RunParamsList>;
       using ValidatedBindState =
           ValidateBindStateType<Traits::is_method, Traits::is_nullable,
                                 Traits::is_callback, Functor, Args...>;
       // This conditional checks if each of the `args` matches to the
       // corresponding param of the target function. This check does not affect
       // the behavior of `Bind()`, but its error message should be more
       // readable.
       if constexpr (std::conjunction_v<
                         NotFunctionRef<Functor>, IsStateless<Traits>,
                         ValidatedUnwrappedTypes,
                         ParamsCanBeBound<
                             Traits::is_method,
                             std::make_index_sequence<sizeof...(Args)>,
                             BoundArgsList,
                             typename ValidatedUnwrappedTypes::Type,
                             BoundParamsList>,
                         ValidatedBindState>) {
         using UnboundRunType =
             MakeFunctionType<ExtractReturnType<typename Traits::RunType>,
                              DropTypeListItem<sizeof...(Args), RunParamsList>>;
         using CallbackType = CallbackT<UnboundRunType>;

         // Store the invoke func into `PolymorphicInvoke` before casting it to
         // `InvokeFuncStorage`, so that we can ensure its type matches to
         // `PolymorphicInvoke`, to which `CallbackType` will cast back.
         typename CallbackType::PolymorphicInvoke invoke_func;
         using Invoker =
             Invoker<Traits, typename ValidatedBindState::Type, UnboundRunType>;
         if constexpr (kIsOnce) {
           invoke_func = Invoker::RunOnce;
         } else {
           invoke_func = Invoker::Run;
         }

         return CallbackType(ValidatedBindState::Type::Create(
             reinterpret_cast<BindStateBase::InvokeFuncStorage>(invoke_func),
             std::forward<Functor>(functor), std::forward<Args>(args)...));
       }
     }
   }

   // Special cases for binding to a `Callback` without extra bound arguments.

   // `OnceCallback` passed to `OnceCallback`, or `RepeatingCallback` passed to
   // `RepeatingCallback`.
   template <typename T>
     requires is_instantiation<T, CallbackT>
   static T BindImpl(T callback) {
     // Guard against null pointers accidentally ending up in posted tasks,
     // causing hard-to-debug crashes.
     CHECK(callback);
     return callback;
   }

   // `RepeatingCallback` passed to `OnceCallback`. The opposite direction is
   // intentionally not supported.
   template <typename Signature>
     requires is_instantiation<OnceCallback<Signature>, CallbackT>
   static OnceCallback<Signature> BindImpl(
       RepeatingCallback<Signature> callback) {
     return BindImpl(OnceCallback<Signature>(callback));
   }

   // Must be defined after `BindImpl()` since it refers to it.
   template <typename Functor, typename... Args>
   struct BindImplWouldSucceed {
     // Can't use a defaulted template param since it can't come after `Args`.
     //
     // Determining if `BindImpl()` would succeed is not as simple as verifying
     // any conditions it checks directly; those only control when it's safe to
     // call other methods, which in turn may fail. However, ultimately, any
     // failure will result in returning `void`, so check for a non-`void` return
     // type.
     static constexpr bool value =
         !std::same_as<void,
                       decltype(BindImpl(std::declval<Functor&&>(),
                                         std::declval<Args&&>()...))>;
   };

  public:
   template <typename Functor, typename... Args>
   static auto Bind(Functor&& functor, Args&&... args) {
     if constexpr (std::conjunction_v<
                       OnceCallbackFunctorIsValid<Functor>,
                       NoBindArgToOnceCallbackIsBasePassed<Args...>,
                       BindImplWouldSucceed<Functor, Args...>>) {
       return BindImpl(std::forward<Functor>(functor),
                       std::forward<Args>(args)...);
     } else {
       return BindFailedCheckPreviousErrors();
     }
   }
 };

 }  // namespace internal

 // An injection point to control `this` pointer behavior on a method invocation.
 // If `IsWeakReceiver<T>::value` is `true` and `T` is used as a method receiver,
 // `Bind()` cancels the method invocation if the receiver tests as false.
 // ```
 //   struct S {
 //     void f() {}
 //   };
 //
 //   WeakPtr<S> weak_s = nullptr;
 //   BindOnce(&S::f, weak_s).Run();  // `S::f()` is not called.
 // ```
 template <typename T>
 struct IsWeakReceiver : std::bool_constant<is_instantiation<T, WeakPtr>> {};

 template <typename T>
 struct IsWeakReceiver<std::reference_wrapper<T>> : IsWeakReceiver<T> {};

 // An injection point to control how objects are checked for maybe validity,
 // which is an optimistic thread-safe check for full validity.
 template <typename>
 struct MaybeValidTraits {
   template <typename T>
   static bool MaybeValid(const T& o) {
     return o.MaybeValid();
   }
 };

 // An injection point to control how bound objects passed to the target
 // function. `BindUnwrapTraits<>::Unwrap()` is called for each bound object
 // right before the target function is invoked.
 template <typename>
 struct BindUnwrapTraits {
   template <typename T>
   static T&& Unwrap(T&& o) {
     return std::forward<T>(o);
   }
 };

 template <typename T>
   requires internal::kIsUnretainedWrapper<internal::UnretainedWrapper, T> ||
            internal::kIsUnretainedWrapper<internal::UnretainedRefWrapper, T> ||
            is_instantiation<T, internal::RetainedRefWrapper> ||
            is_instantiation<T, internal::OwnedWrapper> ||
            is_instantiation<T, internal::OwnedRefWrapper>
 struct BindUnwrapTraits<T> {
   static decltype(auto) Unwrap(const T& o) { return o.get(); }
 };

 template <typename T>
 struct BindUnwrapTraits<internal::PassedWrapper<T>> {
   static T Unwrap(const internal::PassedWrapper<T>& o) { return o.Take(); }
 };

 #if BUILDFLAG(IS_WIN)
 template <typename T>
 struct BindUnwrapTraits<Microsoft::WRL::ComPtr<T>> {
   static T* Unwrap(const Microsoft::WRL::ComPtr<T>& ptr) { return ptr.Get(); }
 };
 #endif

 // `CallbackCancellationTraits` allows customization of `Callback`'s
 // cancellation semantics. By default, callbacks are not cancellable. A
 // specialization should set `is_cancellable` and implement an `IsCancelled()`
 // that returns whether the callback should be cancelled, as well as a
 // `MaybeValid()` that returns whether the underlying functor/object is maybe
 // valid.
 template <typename Functor, typename BoundArgsTuple>
 struct CallbackCancellationTraits {
   static constexpr bool is_cancellable = false;
 };

 // Specialization for a weak receiver.
 template <typename Functor, typename... BoundArgs>
   requires internal::kIsWeakMethod<
       internal::FunctorTraits<Functor, BoundArgs...>::is_method,
       BoundArgs...>
 struct CallbackCancellationTraits<Functor, std::tuple<BoundArgs...>> {
   static constexpr bool is_cancellable = true;

   template <typename Receiver, typename... Args>
   static bool IsCancelled(const Functor&,
                           const Receiver& receiver,
                           const Args&...) {
     return !receiver;
   }

   template <typename Receiver, typename... Args>
   static bool MaybeValid(const Functor&,
                          const Receiver& receiver,
                          const Args&...) {
     return MaybeValidTraits<Receiver>::MaybeValid(receiver);
   }
 };

 // Specialization for a nested `Bind()`.
 template <typename Functor, typename... BoundArgs>
   requires is_instantiation<Functor, OnceCallback> ||
            is_instantiation<Functor, RepeatingCallback>
 struct CallbackCancellationTraits<Functor, std::tuple<BoundArgs...>> {
   static constexpr bool is_cancellable = true;

   static bool IsCancelled(const Functor& functor, const BoundArgs&...) {
     return functor.IsCancelled();
   }

   static bool MaybeValid(const Functor& functor, const BoundArgs&...) {
     return MaybeValidTraits<Functor>::MaybeValid(functor);
   }
 };

 }  // namespace base

 #endif  // BASE_FUNCTIONAL_BIND_INTERNAL_H_