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Editing: invoke.h

// Implementation of INVOKE -*- C++ -*-

// Copyright (C) 2016-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file include/bits/invoke.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{functional}
 */

#ifndef _GLIBCXX_INVOKE_H
#define _GLIBCXX_INVOKE_H 1

#pragma GCC system_header

#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else

#include <type_traits>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
   *  @addtogroup utilities
   *  @{
   */

// Used by __invoke_impl instead of std::forward<_Tp> so that a
  // reference_wrapper is converted to an lvalue-reference.
  template<typename _Tp, typename _Up = typename __inv_unwrap<_Tp>::type>
    constexpr _Up&&
    __invfwd(typename remove_reference<_Tp>::type& __t) noexcept
    { return static_cast<_Up&&>(__t); }

template<typename _Res, typename _Fn, typename... _Args>
    constexpr _Res
    __invoke_impl(__invoke_other, _Fn&& __f, _Args&&... __args)
    { return std::forward<_Fn>(__f)(std::forward<_Args>(__args)...); }

template<typename _Res, typename _MemFun, typename _Tp, typename... _Args>
    constexpr _Res
    __invoke_impl(__invoke_memfun_ref, _MemFun&& __f, _Tp&& __t,
		  _Args&&... __args)
    { return (__invfwd<_Tp>(__t).*__f)(std::forward<_Args>(__args)...); }

template<typename _Res, typename _MemFun, typename _Tp, typename... _Args>
    constexpr _Res
    __invoke_impl(__invoke_memfun_deref, _MemFun&& __f, _Tp&& __t,
		  _Args&&... __args)
    {
      return ((*std::forward<_Tp>(__t)).*__f)(std::forward<_Args>(__args)...);
    }

template<typename _Res, typename _MemPtr, typename _Tp>
    constexpr _Res
    __invoke_impl(__invoke_memobj_ref, _MemPtr&& __f, _Tp&& __t)
    { return __invfwd<_Tp>(__t).*__f; }

template<typename _Res, typename _MemPtr, typename _Tp>
    constexpr _Res
    __invoke_impl(__invoke_memobj_deref, _MemPtr&& __f, _Tp&& __t)
    { return (*std::forward<_Tp>(__t)).*__f; }

/// Invoke a callable object.
  template<typename _Callable, typename... _Args>
    constexpr typename __invoke_result<_Callable, _Args...>::type
    __invoke(_Callable&& __fn, _Args&&... __args)
    noexcept(__is_nothrow_invocable<_Callable, _Args...>::value)
    {
      using __result = __invoke_result<_Callable, _Args...>;
      using __type = typename __result::type;
      using __tag = typename __result::__invoke_type;
      return std::__invoke_impl<__type>(__tag{}, std::forward<_Callable>(__fn),
					std::forward<_Args>(__args)...);
    }

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++11

#endif // _GLIBCXX_INVOKE_H

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