v1/__utility/declval.h

0001 //===----------------------------------------------------------------------===//
0002 //
0003 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0004 // See https://llvm.org/LICENSE.txt for license information.
0005 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0006 //
0007 //===----------------------------------------------------------------------===//
0008
0009 #ifndef _LIBCPP___UTILITY_DECLVAL_H
0010 #define _LIBCPP___UTILITY_DECLVAL_H
0011
0012 #include <__config>
0013
0014 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0015 #  pragma GCC system_header
0016 #endif
0017
0018 _LIBCPP_BEGIN_NAMESPACE_STD
0019
0020 // Suppress deprecation notice for volatile-qualified return type resulting
0021 // from volatile-qualified types _Tp.
0022 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
0023 template <class _Tp>
0024 _Tp&& __declval(int);
0025 template <class _Tp>
0026 _Tp __declval(long);
0027 _LIBCPP_SUPPRESS_DEPRECATED_POP
0028
0029 template <class _Tp>
0030 _LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
0031   static_assert(!__is_same(_Tp, _Tp),
0032                 "std::declval can only be used in an unevaluated context. "
0033                 "It's likely that your current usage is trying to extract a value from the function.");
0034 }
0035
0036 _LIBCPP_END_NAMESPACE_STD
0037
0038 #endif // _LIBCPP___UTILITY_DECLVAL_H