libcxx/include/__utility/forward_like.h

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___UTILITY_FORWARD_LIKE_H
#define _LIBCPP___UTILITY_FORWARD_LIKE_H

#include <__config>
#include <__type_traits/conditional.h>
#include <__type_traits/is_base_of.h>
#include <__type_traits/is_const.h>
#include <__type_traits/is_reference.h>
#include <__type_traits/remove_reference.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 23

template <class _Ap, class _Bp>
using _CopyConst _LIBCPP_NODEBUG = _If<is_const_v<_Ap>, const _Bp, _Bp>;

template <class _Ap, class _Bp>
using _OverrideRef _LIBCPP_NODEBUG = _If<is_rvalue_reference_v<_Ap>, remove_reference_t<_Bp>&&, _Bp&>;

template <class _Ap, class _Bp>
using _ForwardLike _LIBCPP_NODEBUG = _OverrideRef<_Ap&&, _CopyConst<remove_reference_t<_Ap>, remove_reference_t<_Bp>>>;

template <class _Tp, class _Up>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto
forward_like(_LIBCPP_LIFETIMEBOUND _Up&& __ux) noexcept -> _ForwardLike<_Tp, _Up> {
  return static_cast<_ForwardLike<_Tp, _Up>>(__ux);
}

// This function is used for `deducing this` cases where you want to make sure the operation is performed on the class
// itself and not on a derived class. For example
//   struct S {
//     template <class Self>
//     void func(Self&& self) {
//       // This will always call `do_something` of S instead of any class derived from S.
//       std::__forward_as<Self, S>(self).do_something();
//     }
//   };
template <class _Tp, class _As, class _Up>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _ForwardLike<_Tp, _As>
__forward_as(_LIBCPP_LIFETIMEBOUND _Up&& __val) noexcept {
  static_assert(is_base_of_v<_As, remove_reference_t<_Up>>);
  return static_cast<_ForwardLike<_Tp, _As>>(__val);
}

#endif // _LIBCPP_STD_VER >= 23

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___UTILITY_FORWARD_LIKE_H