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>
declval()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