Release absl::AnyInvocable

AnyInvocable is a C++11 compatible equivalent of the C++23 [std::move_only_function](https://en.cppreference.com/w/cpp/utility/functional/move_only_function/move_only_function).
Although this implementation matches an intermediate draft revision of the standard (http://wg21.link/p0288r5), it is neither a standard tracking type nor a seamless backfill type.

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+// Copyright 2022 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: any_invocable.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines an `absl::AnyInvocable` type that assumes ownership
+// and wraps an object of an invocable type. (Invocable types adhere to the
+// concept specified in https://en.cppreference.com/w/cpp/concepts/invocable.)
+//
+// In general, prefer `absl::AnyInvocable` when you need a type-erased
+// function parameter that needs to take ownership of the type.
+//
+// NOTE: `absl::AnyInvocable` is similar to the C++23 `std::move_only_function`
+// abstraction, but has a slightly different API and is not designed to be a
+// drop-in replacement or C++11-compatible backfill of that type.
+
+#ifndef ABSL_FUNCTIONAL_ANY_INVOCABLE_H_
+#define ABSL_FUNCTIONAL_ANY_INVOCABLE_H_
+
+#include <cstddef>
+#include <initializer_list>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/functional/internal/any_invocable.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::AnyInvocable
+//
+// `absl::AnyInvocable` is a functional wrapper type, like `std::function`, that
+// assumes ownership of an invocable object. Unlike `std::function`, an
+// `absl::AnyInvocable` is more type-safe and provides the following additional
+// benefits:
+//
+// * Properly adheres to const correctness of the underlying type
+// * Is move-only so avoids concurrency problems with copied invocables and
+//   unnecessary copies in general.
+// * Supports reference qualifiers allowing it to perform unique actions (noted
+//   below).
+//
+// `absl::AnyInvocable` is a template, and an `absl::AnyInvocable` instantiation
+// may wrap any invocable object with a compatible function signature, e.g.
+// having arguments and return types convertible to types matching the
+// `absl::AnyInvocable` signature, and also matching any stated reference
+// qualifiers, as long as that type is moveable. It therefore provides broad
+// type erasure for functional objects.
+//
+// An `absl::AnyInvocable` is typically used as a type-erased function parameter
+// for accepting various functional objects:
+//
+// // Define a function taking an AnyInvocable parameter.
+// void my_func(absl::AnyInvocable<int()> f) {
+//   ...
+// };
+//
+// // That function can accept any invocable type:
+//
+// // Accept a function reference. We don't need to move a reference.
+// int func1() { return 0; };
+// my_func(func1);
+//
+// // Accept a lambda. We use std::move here because otherwise my_func would
+// // copy the lambda.
+// auto lambda = []() { return 0; };
+// my_func(std::move(lambda));
+//
+// // Accept a function pointer. We don't need to move a function pointer.
+// func2 = &func1;
+// my_func(func2);
+//
+// // Accept an std::function by moving it. Note that the lambda is copyable
+// // (satisfying std::function requirements) and moveable (satisfying
+// // absl::AnyInvocable requirements).
+// std::function<int()> func6 = []() { return 0; };
+// my_func(std::move(func6));
+//
+// `AnyInvocable` also properly respects `const` qualifiers, reference
+// qualifiers, and the `noexcept` specification (only in C++ 17 and beyond) as
+// part of the user-specified function type (e.g.
+// `AnyInvocable<void()&& const noexcept>`). These qualifiers will be applied to
+// the `AnyInvocable` object's `operator()`, and the underlying invocable must
+// be compatible with those qualifiers.
+//
+// Comparison of const and non-const function types:
+//
+//   // Store a closure inside of `func` with the function type `int()`.
+//   // Note that we have made `func` itself `const`.
+//   const AnyInvocable<int()> func = [](){ return 0; };
+//
+//   func();  // Compile-error: the passed type `int()` isn't `const`.
+//
+//   // Store a closure inside of `const_func` with the function type
+//   // `int() const`.
+//   // Note that we have also made `const_func` itself `const`.
+//   const AnyInvocable<int() const> const_func = [](){ return 0; };
+//
+//   const_func();  // Fine: `int() const` is `const`.
+//
+// In the above example, the call `func()` would have compiled if
+// `std::function` were used even though the types are not const compatible.
+// This is a bug, and using `absl::AnyInvocable` properly detects that bug.
+//
+// In addition to affecting the signature of `operator()`, the `const` and
+// reference qualifiers of the function type also appropriately constrain which
+// kinds of invocable objects you are allowed to place into the `AnyInvocable`
+// instance. If you specify a function type that is const-qualified, then
+// anything that you attempt to put into the `AnyInvocable` must be callable on
+// a `const` instance of that type.
+//
+// Constraint example:
+//
+//   // Fine because the lambda is callable when `const`.
+//   AnyInvocable<int() const> func = [=](){ return 0; };
+//
+//   // This is a compile-error because the lambda isn't callable when `const`.
+//   AnyInvocable<int() const> error = [=]() mutable { return 0; };
+//
+// An `&&` qualifier can be used to express that an `absl::AnyInvocable`
+// instance should be invoked at most once:
+//
+//   // Invokes `continuation` with the logical result of an operation when
+//   // that operation completes (common in asynchronous code).
+//   void CallOnCompletion(AnyInvocable<void(int)&&> continuation) {
+//     int result_of_foo = foo();
+//
+//     // `std::move` is required because the `operator()` of `continuation` is
+//     // rvalue-reference qualified.
+//     std::move(continuation)(result_of_foo);
+//   }
+//
+// Credits to Matt Calabrese (https://github.com/mattcalabrese) for the original
+// implementation.
+template <class Sig>
+class AnyInvocable : private internal_any_invocable::Impl<Sig> {
+ private:
+  static_assert(
+      std::is_function<Sig>::value,
+      "The template argument of AnyInvocable must be a function type.");
+
+  using Impl = internal_any_invocable::Impl<Sig>;
+
+ public:
+  // The return type of Sig
+  using result_type = typename Impl::result_type;
+
+  // Constructors
+
+  // Constructs the `AnyInvocable` in an empty state.
+  AnyInvocable() noexcept = default;
+  AnyInvocable(std::nullptr_t) noexcept {}  // NOLINT
+
+  // Constructs the `AnyInvocable` from an existing `AnyInvocable` by a move.
+  // Note that `f` is not guaranteed to be empty after move-construction,
+  // although it may be.
+  AnyInvocable(AnyInvocable&& /*f*/) noexcept = default;
+
+  // Constructs an `AnyInvocable` from an invocable object.
+  //
+  // Upon construction, `*this` is only empty if `f` is a function pointer or
+  // member pointer type and is null, or if `f` is an `AnyInvocable` that is
+  // empty.
+  template <class F, typename = absl::enable_if_t<
+                         internal_any_invocable::CanConvert<Sig, F>::value>>
+  AnyInvocable(F&& f)  // NOLINT
+      : Impl(internal_any_invocable::ConversionConstruct(),
+             std::forward<F>(f)) {}
+
+  // Constructs an `AnyInvocable` that holds an invocable object of type `T`,
+  // which is constructed in-place from the given arguments.
+  //
+  // Example:
+  //
+  //   AnyInvocable<int(int)> func(
+  //       absl::in_place_type<PossiblyImmovableType>, arg1, arg2);
+  //
+  template <class T, class... Args,
+            typename = absl::enable_if_t<
+                internal_any_invocable::CanEmplace<Sig, T, Args...>::value>>
+  explicit AnyInvocable(absl::in_place_type_t<T>, Args&&... args)
+      : Impl(absl::in_place_type<absl::decay_t<T>>,
+             std::forward<Args>(args)...) {
+    static_assert(std::is_same<T, absl::decay_t<T>>::value,
+                  "The explicit template argument of in_place_type is required "
+                  "to be an unqualified object type.");
+  }
+
+  // Overload of the above constructor to support list-initialization.
+  template <class T, class U, class... Args,
+            typename = absl::enable_if_t<internal_any_invocable::CanEmplace<
+                Sig, T, std::initializer_list<U>&, Args...>::value>>
+  explicit AnyInvocable(absl::in_place_type_t<T>,
+                        std::initializer_list<U> ilist, Args&&... args)
+      : Impl(absl::in_place_type<absl::decay_t<T>>, ilist,
+             std::forward<Args>(args)...) {
+    static_assert(std::is_same<T, absl::decay_t<T>>::value,
+                  "The explicit template argument of in_place_type is required "
+                  "to be an unqualified object type.");
+  }
+
+  // Assignment Operators
+
+  // Assigns an `AnyInvocable` through move-assignment.
+  // Note that `f` is not guaranteed to be empty after move-assignment
+  // although it may be.
+  AnyInvocable& operator=(AnyInvocable&& /*f*/) noexcept = default;
+
+  // Assigns an `AnyInvocable` from a nullptr, clearing the `AnyInvocable`. If
+  // not empty, destroys the target, putting `*this` into an empty state.
+  AnyInvocable& operator=(std::nullptr_t) noexcept {
+    this->Clear();
+    return *this;
+  }
+
+  // Assigns an `AnyInvocable` from an existing `AnyInvocable` instance.
+  //
+  // Upon assignment, `*this` is only empty if `f` is a function pointer or
+  // member pointer type and is null, or if `f` is an `AnyInvocable` that is
+  // empty.
+  template <class F, typename = absl::enable_if_t<
+                         internal_any_invocable::CanAssign<Sig, F>::value>>
+  AnyInvocable& operator=(F&& f) {
+    *this = AnyInvocable(std::forward<F>(f));
+    return *this;
+  }
+
+  // Assigns an `AnyInvocable` from a reference to an invocable object.
+  // Upon assignment, stores a reference to the invocable object in the
+  // `AnyInvocable` instance.
+  template <
+      class F,
+      typename = absl::enable_if_t<
+          internal_any_invocable::CanAssignReferenceWrapper<Sig, F>::value>>
+  AnyInvocable& operator=(std::reference_wrapper<F> f) noexcept {
+    *this = AnyInvocable(f);
+    return *this;
+  }
+
+  // Destructor
+
+  // If not empty, destroys the target.
+  ~AnyInvocable() = default;
+
+  // absl::AnyInvocable::swap()
+  //
+  // Exchanges the targets of `*this` and `other`.
+  void swap(AnyInvocable& other) noexcept { std::swap(*this, other); }
+
+  // abl::AnyInvocable::operator bool()
+  //
+  // Returns `true` if `*this` is not empty.
+  explicit operator bool() const noexcept { return this->HasValue(); }
+
+  // Invokes the target object of `*this`. `*this` must not be empty.
+  //
+  // Note: The signature of this function call operator is the same as the
+  //       template parameter `Sig`.
+  using Impl::operator();
+
+  // Equality operators
+
+  // Returns `true` if `*this` is empty.
+  friend bool operator==(const AnyInvocable& f, std::nullptr_t) noexcept {
+    return !f.HasValue();
+  }
+
+  // Returns `true` if `*this` is empty.
+  friend bool operator==(std::nullptr_t, const AnyInvocable& f) noexcept {
+    return !f.HasValue();
+  }
+
+  // Returns `false` if `*this` is empty.
+  friend bool operator!=(const AnyInvocable& f, std::nullptr_t) noexcept {
+    return f.HasValue();
+  }
+
+  // Returns `false` if `*this` is empty.
+  friend bool operator!=(std::nullptr_t, const AnyInvocable& f) noexcept {
+    return f.HasValue();
+  }
+
+  // swap()
+  //
+  // Exchanges the targets of `f1` and `f2`.
+  friend void swap(AnyInvocable& f1, AnyInvocable& f2) noexcept { f1.swap(f2); }
+
+ private:
+  // Friending other instantiations is necessary for conversions.
+  template <bool /*SigIsNoexcept*/, class /*ReturnType*/, class... /*P*/>
+  friend class internal_any_invocable::CoreImpl;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_ANY_INVOCABLE_H_