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
23template <class _Tp>
24_Tp&& __declval(int);
25template <class _Tp>
26_Tp __declval(long);
27_LIBCPP_SUPPRESS_DEPRECATED_POP
28
29template <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