base/bind_internal.h

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

#ifndef BASE_BIND_INTERNAL_H_
#define BASE_BIND_INTERNAL_H_

#include <stddef.h>

#include <type_traits>
#include <utility>

#include "base/callback_internal.h"
#include "base/compiler_specific.h"
#include "base/memory/raw_scoped_refptr_mismatch_checker.h"
#include "base/memory/weak_ptr.h"
#include "base/template_util.h"
#include "build/build_config.h"

#if defined(OS_MACOSX) && !HAS_FEATURE(objc_arc)
#include "base/mac/scoped_block.h"
#endif

// See base/callback.h 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.
//  InvokeHelper<> -- Take a Functor + arguments and actully invokes it.
//                    Handle the differing syntaxes needed for WeakPtr<>
//                    support.  This is separate from Invoker to avoid creating
//                    multiple version 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 defined(OS_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, typename SFINAE = void>
struct CallbackCancellationTraits;

namespace internal {

template <typename Functor, typename SFINAE = void>
struct FunctorTraits;

template <typename T>
class UnretainedWrapper {
 public:
  explicit UnretainedWrapper(T* o) : ptr_(o) {}
  T* get() const { return ptr_; }

 private:
  T* ptr_;
};

template <typename T>
class ConstRefWrapper {
 public:
  explicit ConstRefWrapper(const T& o) : ptr_(&o) {}
  const T& get() const { return *ptr_; }

 private:
  const T* ptr_;
};

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_;
};

// An alternate implementation is to avoid the destructive copy, and instead
// specialize ParamTraits<> for OwnedWrapper<> to change the StorageType to
// a class that is essentially a std::unique_ptr<>.
//
// The current implementation has the benefit though of leaving ParamTraits<>
// fully in callback_internal.h as well as avoiding type conversions during
// storage.
template <typename T>
class OwnedWrapper {
 public:
  explicit OwnedWrapper(T* o) : ptr_(o) {}
  ~OwnedWrapper() { delete ptr_; }
  T* get() const { return ptr_; }
  OwnedWrapper(OwnedWrapper&& other) {
    ptr_ = other.ptr_;
    other.ptr_ = NULL;
  }

 private:
  mutable T* ptr_;
};

// 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 C++11 rvalue references or
// a 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 whitelisted 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 NULL because it is valid to bind a "NULL"
//     scoper to a Callback and allow the Callback to execute once.
template <typename T>
class PassedWrapper {
 public:
  explicit PassedWrapper(T&& scoper)
      : is_valid_(true), 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_;
  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));
}

// IsWeakMethod is a helper that determine 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>
struct IsWeakMethod : std::false_type {};

template <typename T, typename... Args>
struct IsWeakMethod<true, T, Args...> : IsWeakReceiver<T> {};

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

// Used for DropTypeListItem implementation.
template <size_t n, typename List>
struct DropTypeListItemImpl;

// Do not use enable_if and SFINAE here to avoid MSVC2013 compile failure.
template <size_t n, typename T, typename... List>
struct DropTypeListItemImpl<n, TypeList<T, List...>>
    : DropTypeListItemImpl<n - 1, TypeList<List...>> {};

template <typename T, typename... List>
struct DropTypeListItemImpl<0, TypeList<T, List...>> {
  using Type = TypeList<T, List...>;
};

template <>
struct DropTypeListItemImpl<0, TypeList<>> {
  using Type = TypeList<>;
};

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

// Used for TakeTypeListItem implementation.
template <size_t n, typename List, typename... Accum>
struct TakeTypeListItemImpl;

// Do not use enable_if and SFINAE here to avoid MSVC2013 compile failure.
template <size_t n, typename T, typename... List, typename... Accum>
struct TakeTypeListItemImpl<n, TypeList<T, List...>, Accum...>
    : TakeTypeListItemImpl<n - 1, TypeList<List...>, Accum..., T> {};

template <typename T, typename... List, typename... Accum>
struct TakeTypeListItemImpl<0, TypeList<T, List...>, Accum...> {
  using Type = TypeList<Accum...>;
};

template <typename... Accum>
struct TakeTypeListItemImpl<0, TypeList<>, Accum...> {
  using Type = TypeList<Accum...>;
};

// A type-level function that takes first |n| list item from given TypeList.
// E.g. TakeTypeListItem<3, TypeList<A, B, C, D>> is evaluated to
// TypeList<A, B, C>.
template <size_t n, typename List>
using TakeTypeListItem = typename TakeTypeListItemImpl<n, List>::Type;

// Used for ConcatTypeLists implementation.
template <typename List1, typename List2>
struct ConcatTypeListsImpl;

template <typename... Types1, typename... Types2>
struct ConcatTypeListsImpl<TypeList<Types1...>, TypeList<Types2...>> {
  using Type = TypeList<Types1..., Types2...>;
};

// A type-level function that concats two TypeLists.
template <typename List1, typename List2>
using ConcatTypeLists = typename ConcatTypeListsImpl<List1, List2>::Type;

// Used for MakeFunctionType implementation.
template <typename R, typename ArgList>
struct MakeFunctionTypeImpl;

template <typename R, typename... Args>
struct MakeFunctionTypeImpl<R, TypeList<Args...>> {
  // MSVC 2013 doesn't support Type Alias of function types.
  // Revisit this after we update it to newer version.
  typedef R Type(Args...);
};

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

// Used for 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)> is evaluated to 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)> is evaluated to R.
template <typename Signature>
using ExtractReturnType = typename ExtractArgsImpl<Signature>::ReturnType;

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

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

template <typename Callable, typename R, typename... Args>
struct ExtractCallableRunTypeImpl<Callable, R (Callable::*)(Args...) const> {
  using Type = R(Args...);
};

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

// IsCallableObject<Functor> is std::true_type if |Functor| has operator().
// Otherwise, it's std::false_type.
// Example:
//   IsCallableObject<void(*)()>::value is false.
//
//   struct Foo {};
//   IsCallableObject<void(Foo::*)()>::value is false.
//
//   int i = 0;
//   auto f = [i]() {};
//   IsCallableObject<decltype(f)>::value is false.
template <typename Functor, typename SFINAE = void>
struct IsCallableObject : std::false_type {};

template <typename Callable>
struct IsCallableObject<Callable, void_t<decltype(&Callable::operator())>>
    : std::true_type {};

// HasRefCountedTypeAsRawPtr selects true_type when any of the |Args| is a raw
// pointer to a RefCounted type.
// Implementation note: This non-specialized case handles zero-arity case only.
// Non-zero-arity cases should be handled by the specialization below.
template <typename... Args>
struct HasRefCountedTypeAsRawPtr : std::false_type {};

// Implementation note: Select true_type if the first parameter is a raw pointer
// to a RefCounted type. Otherwise, skip the first parameter and check rest of
// parameters recursively.
template <typename T, typename... Args>
struct HasRefCountedTypeAsRawPtr<T, Args...>
    : std::conditional_t<NeedsScopedRefptrButGetsRawPtr<T>::value,
                         std::true_type,
                         HasRefCountedTypeAsRawPtr<Args...>> {};

// ForceVoidReturn<>
//
// Set of templates that support forcing the function return type to void.
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.
template <typename Functor, typename SFINAE>
struct FunctorTraits;

// For empty callable types.
// This specialization is intended to allow binding captureless lambdas by
// base::Bind(), based on the fact that captureless lambdas are empty while
// capturing lambdas are not. This also allows any functors as far as it's an
// empty class.
// Example:
//
//   // Captureless lambdas are allowed.
//   []() {return 42;};
//
//   // Capturing lambdas are *not* allowed.
//   int x;
//   [x]() {return x;};
//
//   // Any empty class with operator() is allowed.
//   struct Foo {
//     void operator()() const {}
//     // No non-static member variable and no virtual functions.
//   };
template <typename Functor>
struct FunctorTraits<Functor,
                     std::enable_if_t<IsCallableObject<Functor>::value &&
                                      std::is_empty<Functor>::value>> {
  using RunType = ExtractCallableRunType<Functor>;
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = false;

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

// For functions.
template <typename R, typename... Args>
struct FunctorTraits<R (*)(Args...)> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = true;

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

#if defined(OS_WIN) && !defined(ARCH_CPU_X86_64)

// For functions.
template <typename R, typename... Args>
struct FunctorTraits<R(__stdcall*)(Args...)> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = true;

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

// For functions.
template <typename R, typename... Args>
struct FunctorTraits<R(__fastcall*)(Args...)> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = true;

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

#endif  // defined(OS_WIN) && !defined(ARCH_CPU_X86_64)

#if defined(OS_MACOSX)

// Support for Objective-C blocks. There are two implementation depending
// on whether Automated Reference Counting (ARC) is enabled. When ARC is
// enabled, then the block itself can be bound as the compiler will ensure
// its lifetime will be correctly managed. Otherwise, require the block to
// be wrapped in a base::mac::ScopedBlock (via base::RetainBlock) that will
// correctly manage the block lifetime.
//
// The two implementation ensure that the One Definition Rule (ODR) is not
// broken (it is not possible to write a template base::RetainBlock that would
// work correctly both with ARC enabled and disabled).

#if HAS_FEATURE(objc_arc)

template <typename R, typename... Args>
struct FunctorTraits<R (^)(Args...)> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = 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)...);
  }
};

#else  // HAS_FEATURE(objc_arc)

template <typename R, typename... Args>
struct FunctorTraits<base::mac::ScopedBlock<R (^)(Args...)>> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = true;

  template <typename BlockType, typename... RunArgs>
  static R Invoke(BlockType&& block, RunArgs&&... args) {
    // Copy the block 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.
    base::mac::ScopedBlock<R (^)(Args...)> scoped_block(block);
    return scoped_block.get()(std::forward<RunArgs>(args)...);
  }
};

#endif  // HAS_FEATURE(objc_arc)
#endif  // defined(OS_MACOSX)

// For methods.
template <typename R, typename Receiver, typename... Args>
struct FunctorTraits<R (Receiver::*)(Args...)> {
  using RunType = R(Receiver*, Args...);
  static constexpr bool is_method = true;
  static constexpr bool is_nullable = 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)...);
  }
};

// For const methods.
template <typename R, typename Receiver, typename... Args>
struct FunctorTraits<R (Receiver::*)(Args...) const> {
  using RunType = R(const Receiver*, Args...);
  static constexpr bool is_method = true;
  static constexpr bool is_nullable = 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)...);
  }
};

#ifdef __cpp_noexcept_function_type
// noexcept makes a distinct function type in C++17.
// I.e. `void(*)()` and `void(*)() noexcept` are same in pre-C++17, and
// different in C++17.
template <typename R, typename... Args>
struct FunctorTraits<R (*)(Args...) noexcept> : FunctorTraits<R (*)(Args...)> {
};

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

template <typename R, typename Receiver, typename... Args>
struct FunctorTraits<R (Receiver::*)(Args...) const noexcept>
    : FunctorTraits<R (Receiver::*)(Args...) const> {};
#endif

// For IgnoreResults.
template <typename T>
struct FunctorTraits<IgnoreResultHelper<T>> : FunctorTraits<T> {
  using RunType =
      typename ForceVoidReturn<typename FunctorTraits<T>::RunType>::RunType;

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

// For OnceCallbacks.
template <typename R, typename... Args>
struct FunctorTraits<OnceCallback<R(Args...)>> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = 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 RepeatingCallbacks.
template <typename R, typename... Args>
struct FunctorTraits<RepeatingCallback<R(Args...)>> {
  using RunType = R(Args...);
  static constexpr bool is_method = false;
  static constexpr bool is_nullable = 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)...);
  }
};

template <typename Functor>
using MakeFunctorTraits = FunctorTraits<std::decay_t<Functor>>;

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

template <typename ReturnType>
struct InvokeHelper<false, ReturnType> {
  template <typename Functor, typename... RunArgs>
  static inline ReturnType MakeItSo(Functor&& functor, RunArgs&&... args) {
    using Traits = MakeFunctorTraits<Functor>;
    return Traits::Invoke(std::forward<Functor>(functor),
                          std::forward<RunArgs>(args)...);
  }
};

template <typename ReturnType>
struct InvokeHelper<true, ReturnType> {
  // WeakCalls are only supported for functions with a void return type.
  // Otherwise, the function result would be undefined if the the WeakPtr<>
  // is invalidated.
  static_assert(std::is_void<ReturnType>::value,
                "weak_ptrs can only bind to methods without return values");

  template <typename Functor, typename BoundWeakPtr, typename... RunArgs>
  static inline void MakeItSo(Functor&& functor,
                              BoundWeakPtr&& weak_ptr,
                              RunArgs&&... args) {
    if (!weak_ptr)
      return;
    using Traits = MakeFunctorTraits<Functor>;
    Traits::Invoke(std::forward<Functor>(functor),
                   std::forward<BoundWeakPtr>(weak_ptr),
                   std::forward<RunArgs>(args)...);
  }
};

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

template <typename StorageType, typename R, typename... UnboundArgs>
struct Invoker<StorageType, R(UnboundArgs...)> {
  static R RunOnce(BindStateBase* base,
                   PassingType<UnboundArgs>... unbound_args) {
    // Local references to make debugger stepping easier. If in a debugger,
    // you really want to warp ahead and step through the
    // InvokeHelper<>::MakeItSo() call below.
    StorageType* storage = static_cast<StorageType*>(base);
    static constexpr size_t num_bound_args =
        std::tuple_size<decltype(storage->bound_args_)>::value;
    return RunImpl(std::move(storage->functor_),
                   std::move(storage->bound_args_),
                   std::make_index_sequence<num_bound_args>(),
                   std::forward<UnboundArgs>(unbound_args)...);
  }

  static R Run(BindStateBase* base, PassingType<UnboundArgs>... unbound_args) {
    // Local references to make debugger stepping easier. If in a debugger,
    // you really want to warp ahead and step through the
    // InvokeHelper<>::MakeItSo() call below.
    const StorageType* storage = static_cast<StorageType*>(base);
    static constexpr size_t num_bound_args =
        std::tuple_size<decltype(storage->bound_args_)>::value;
    return RunImpl(storage->functor_, storage->bound_args_,
                   std::make_index_sequence<num_bound_args>(),
                   std::forward<UnboundArgs>(unbound_args)...);
  }

 private:
  template <typename Functor, typename BoundArgsTuple, size_t... indices>
  static inline R RunImpl(Functor&& functor,
                          BoundArgsTuple&& bound,
                          std::index_sequence<indices...>,
                          UnboundArgs&&... unbound_args) {
    static constexpr bool is_method = MakeFunctorTraits<Functor>::is_method;

    using DecayedArgsTuple = std::decay_t<BoundArgsTuple>;
    static constexpr bool is_weak_call =
        IsWeakMethod<is_method,
                     std::tuple_element_t<indices, DecayedArgsTuple>...>();

    return InvokeHelper<is_weak_call, R>::MakeItSo(
        std::forward<Functor>(functor),
        Unwrap(std::get<indices>(std::forward<BoundArgsTuple>(bound)))...,
        std::forward<UnboundArgs>(unbound_args)...);
  }
};

// Extracts necessary type info from Functor and BoundArgs.
// Used to implement MakeUnboundRunType, BindOnce and BindRepeating.
template <typename Functor, typename... BoundArgs>
struct BindTypeHelper {
  static constexpr size_t num_bounds = sizeof...(BoundArgs);
  using FunctorTraits = MakeFunctorTraits<Functor>;

  // Example:
  //   When Functor is `double (Foo::*)(int, const std::string&)`, and BoundArgs
  //   is a template pack of `Foo*` and `int16_t`:
  //    - RunType is `double(Foo*, int, const std::string&)`,
  //    - ReturnType is `double`,
  //    - RunParamsList is `TypeList<Foo*, int, const std::string&>`,
  //    - BoundParamsList is `TypeList<Foo*, int>`,
  //    - UnboundParamsList is `TypeList<const std::string&>`,
  //    - BoundArgsList is `TypeList<Foo*, int16_t>`,
  //    - UnboundRunType is `double(const std::string&)`.
  using RunType = typename FunctorTraits::RunType;
  using ReturnType = ExtractReturnType<RunType>;

  using RunParamsList = ExtractArgs<RunType>;
  using BoundParamsList = TakeTypeListItem<num_bounds, RunParamsList>;
  using UnboundParamsList = DropTypeListItem<num_bounds, RunParamsList>;

  using BoundArgsList = TypeList<BoundArgs...>;

  using UnboundRunType = MakeFunctionType<ReturnType, UnboundParamsList>;
};

template <typename Functor>
std::enable_if_t<FunctorTraits<Functor>::is_nullable, bool> IsNull(
    const Functor& functor) {
  return !functor;
}

template <typename Functor>
std::enable_if_t<!FunctorTraits<Functor>::is_nullable, bool> IsNull(
    const Functor&) {
  return false;
}

// Used by ApplyCancellationTraits below.
template <typename Functor, typename BoundArgsTuple, size_t... indices>
bool ApplyCancellationTraitsImpl(const Functor& functor,
                                 const BoundArgsTuple& bound_args,
                                 std::index_sequence<indices...>) {
  return CallbackCancellationTraits<Functor, BoundArgsTuple>::IsCancelled(
      functor, std::get<indices>(bound_args)...);
}

// Relays |base| to corresponding CallbackCancellationTraits<>::Run(). Returns
// true if the callback |base| represents is canceled.
template <typename BindStateType>
bool ApplyCancellationTraits(const BindStateBase* base) {
  const BindStateType* storage = static_cast<const BindStateType*>(base);
  static constexpr size_t num_bound_args =
      std::tuple_size<decltype(storage->bound_args_)>::value;
  return ApplyCancellationTraitsImpl(
      storage->functor_, storage->bound_args_,
      std::make_index_sequence<num_bound_args>());
};

// BindState<>
//
// This stores all the state passed into Bind().
template <typename Functor, typename... BoundArgs>
struct BindState final : BindStateBase {
  using IsCancellable = std::integral_constant<
      bool,
      CallbackCancellationTraits<Functor,
                                 std::tuple<BoundArgs...>>::is_cancellable>;

  template <typename ForwardFunctor, typename... ForwardBoundArgs>
  explicit BindState(BindStateBase::InvokeFuncStorage invoke_func,
                     ForwardFunctor&& functor,
                     ForwardBoundArgs&&... bound_args)
      // IsCancellable is std::false_type if
      // CallbackCancellationTraits<>::IsCancelled returns always false.
      // Otherwise, it's std::true_type.
      : BindState(IsCancellable{},
                  invoke_func,
                  std::forward<ForwardFunctor>(functor),
                  std::forward<ForwardBoundArgs>(bound_args)...) {}

  Functor functor_;
  std::tuple<BoundArgs...> bound_args_;

 private:
  template <typename ForwardFunctor, typename... ForwardBoundArgs>
  explicit BindState(std::true_type,
                     BindStateBase::InvokeFuncStorage invoke_func,
                     ForwardFunctor&& functor,
                     ForwardBoundArgs&&... bound_args)
      : BindStateBase(invoke_func,
                      &Destroy,
                      &ApplyCancellationTraits<BindState>),
        functor_(std::forward<ForwardFunctor>(functor)),
        bound_args_(std::forward<ForwardBoundArgs>(bound_args)...) {
    DCHECK(!IsNull(functor_));
  }

  template <typename ForwardFunctor, typename... ForwardBoundArgs>
  explicit BindState(std::false_type,
                     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)...) {
    DCHECK(!IsNull(functor_));
  }

  ~BindState() = default;

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

// Used to implement MakeBindStateType.
template <bool is_method, typename Functor, typename... BoundArgs>
struct MakeBindStateTypeImpl;

template <typename Functor, typename... BoundArgs>
struct MakeBindStateTypeImpl<false, Functor, BoundArgs...> {
  static_assert(!HasRefCountedTypeAsRawPtr<std::decay_t<BoundArgs>...>::value,
                "A parameter is a refcounted type and needs scoped_refptr.");
  using Type = BindState<std::decay_t<Functor>, std::decay_t<BoundArgs>...>;
};

template <typename Functor>
struct MakeBindStateTypeImpl<true, Functor> {
  using Type = BindState<std::decay_t<Functor>>;
};

template <typename Functor, typename Receiver, typename... BoundArgs>
struct MakeBindStateTypeImpl<true, Functor, Receiver, BoundArgs...> {
 private:
  using DecayedReceiver = std::decay_t<Receiver>;

  static_assert(!std::is_array<std::remove_reference_t<Receiver>>::value,
                "First bound argument to a method cannot be an array.");
  static_assert(
      !std::is_pointer<DecayedReceiver>::value ||
          IsRefCountedType<std::remove_pointer_t<DecayedReceiver>>::value,
      "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.");
  static_assert(!HasRefCountedTypeAsRawPtr<std::decay_t<BoundArgs>...>::value,
                "A parameter is a refcounted type and needs scoped_refptr.");

 public:
  using Type = BindState<
      std::decay_t<Functor>,
      std::conditional_t<std::is_pointer<DecayedReceiver>::value,
                         scoped_refptr<std::remove_pointer_t<DecayedReceiver>>,
                         DecayedReceiver>,
      std::decay_t<BoundArgs>...>;
};

template <typename Functor, typename... BoundArgs>
using MakeBindStateType =
    typename MakeBindStateTypeImpl<MakeFunctorTraits<Functor>::is_method,
                                   Functor,
                                   BoundArgs...>::Type;

}  // namespace internal

// An injection point to control |this| pointer behavior on a method invocation.
// If IsWeakReceiver<> is true_type for |T| and |T| is used for a receiver of a
// method, base::Bind cancels the method invocation if the receiver is tested as
// false.
// E.g. Foo::bar() is not called:
//   struct Foo : base::SupportsWeakPtr<Foo> {
//     void bar() {}
//   };
//
//   WeakPtr<Foo> oo = nullptr;
//   base::Bind(&Foo::bar, oo).Run();
template <typename T>
struct IsWeakReceiver : std::false_type {};

template <typename T>
struct IsWeakReceiver<internal::ConstRefWrapper<T>> : IsWeakReceiver<T> {};

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

// An injection point to control how bound objects passed to the target
// function. BindUnwrapTraits<>::Unwrap() is called for each bound objects 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>
struct BindUnwrapTraits<internal::UnretainedWrapper<T>> {
  static T* Unwrap(const internal::UnretainedWrapper<T>& o) { return o.get(); }
};

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

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

template <typename T>
struct BindUnwrapTraits<internal::OwnedWrapper<T>> {
  static T* Unwrap(const internal::OwnedWrapper<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 defined(OS_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 = true and implement an IsCancelled() that returns if the
// callback should be cancelled.
template <typename Functor, typename BoundArgsTuple, typename SFINAE>
struct CallbackCancellationTraits {
  static constexpr bool is_cancellable = false;
};

// Specialization for method bound to weak pointer receiver.
template <typename Functor, typename... BoundArgs>
struct CallbackCancellationTraits<
    Functor,
    std::tuple<BoundArgs...>,
    std::enable_if_t<
        internal::IsWeakMethod<internal::FunctorTraits<Functor>::is_method,
                               BoundArgs...>::value>> {
  static constexpr bool is_cancellable = true;

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

// Specialization for a nested bind.
template <typename Signature, typename... BoundArgs>
struct CallbackCancellationTraits<OnceCallback<Signature>,
                                  std::tuple<BoundArgs...>> {
  static constexpr bool is_cancellable = true;

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

template <typename Signature, typename... BoundArgs>
struct CallbackCancellationTraits<RepeatingCallback<Signature>,
                                  std::tuple<BoundArgs...>> {
  static constexpr bool is_cancellable = true;

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

// Returns a RunType of bound functor.
// E.g. MakeUnboundRunType<R(A, B, C), A, B> is evaluated to R(C).
template <typename Functor, typename... BoundArgs>
using MakeUnboundRunType =
    typename internal::BindTypeHelper<Functor, BoundArgs...>::UnboundRunType;

}  // namespace base

#endif  // BASE_BIND_INTERNAL_H_