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 39