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1// Copyright (c) 2019-2020 Antony Polukhin.
2//
3// Distributed under the Boost Software License, Version 1.0. (See accompanying
4// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
5
6#ifndef BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
7#define BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
8
9#include <boost/pfr/detail/config.hpp>
10
11#include <type_traits>
12
13namespace boost { namespace pfr { namespace detail {
14
15// This function serves as a link-time assert. If linker requires it, then
16// `unsafe_declval()` is used at runtime.
17void report_if_you_see_link_error_with_this_function() noexcept;
18
19// For returning non default constructible types. Do NOT use at runtime!
20//
21// GCCs std::declval may not be used in potentionally evaluated contexts,
22// so we reinvent it.
23template <class T>
24constexpr T unsafe_declval() noexcept {
25 report_if_you_see_link_error_with_this_function();
26
27 typename std::remove_reference<T>::type* ptr = 0;
28 ptr += 42; // suppresses 'null pointer dereference' warnings
29 return static_cast<T>(*ptr);
30}
31
32}}} // namespace boost::pfr::detail
33
34
35#endif // BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
36