libcxx/include/__utility/declval.h

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #ifndef _LIBCPP___UTILITY_DECLVAL_H
  10 #define _LIBCPP___UTILITY_DECLVAL_H
  11
  12 #include <__config>
  13
  14 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  15 #  pragma GCC system_header
  16 #endif
  17
  18 _LIBCPP_BEGIN_NAMESPACE_STD
  19
  20 // Suppress deprecation notice for volatile-qualified return type resulting
  21 // from volatile-qualified types _Tp.
  22 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
  23 template <class _Tp>
  24 _Tp&& __declval(int);
  25 template <class _Tp>
  26 _Tp __declval(long);
  27 _LIBCPP_SUPPRESS_DEPRECATED_POP
  28
  29 template <class _Tp>
  30 _LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
  31   static_assert(!__is_same(_Tp, _Tp),
  32                 "std::declval can only be used in an unevaluated context. "
  33                 "It's likely that your current usage is trying to extract a value from the function.");
  34 }
  35
  36 _LIBCPP_END_NAMESPACE_STD
  37
  38 #endif // _LIBCPP___UTILITY_DECLVAL_H