整理

include/boost/pfr/detail/fake_object.hpp

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+// Copyright (c) 2023 Bela Schaum, X-Ryl669, Denis Mikhailov.
+// Copyright (c) 2024-2025 Antony Polukhin
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+
+// Initial implementation by Bela Schaum, https://github.com/schaumb
+// The way to make it union and UB free by X-Ryl669, https://github.com/X-Ryl669
+//
+
+#ifndef BOOST_PFR_DETAIL_FAKE_OBJECT_HPP
+#define BOOST_PFR_DETAIL_FAKE_OBJECT_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#ifdef __clang__
+#   pragma clang diagnostic push
+#   pragma clang diagnostic ignored "-Wundefined-internal"
+#   pragma clang diagnostic ignored "-Wundefined-var-template"
+#endif
+
+namespace boost { namespace pfr { namespace detail {
+
+// This class has external linkage while T has not sure.
+template <class T>
+struct wrapper {
+    const T value;
+};
+
+// This variable servers as a link-time assert.
+// If linker requires it, then `fake_object()` is used at runtime.
+template <class T>
+extern const wrapper<T> do_not_use_PFR_with_local_types;
+
+// For returning non default constructible types, it's exclusively used in member name retrieval.
+//
+// Neither std::declval nor boost::pfr::detail::unsafe_declval are usable there.
+// This takes advantage of C++20 features, while boost::pfr::detail::unsafe_declval works
+// with the former standards.
+template <class T>
+constexpr const T& fake_object() noexcept {
+    return do_not_use_PFR_with_local_types<T>.value;
+}
+
+}}} // namespace boost::pfr::detail
+
+#ifdef __clang__
+#   pragma clang diagnostic pop
+#endif
+
+#endif // BOOST_PFR_DETAIL_FAKE_OBJECT_HPP
+