整理

include/boost/pfr/detail/unsafe_declval.hpp

@@ -0,0 +1,49 @@
+// Copyright (c) 2019-2025 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)
+
+#ifndef BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
+#define BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
+#pragma once
+
+#include <boost/pfr/detail/config.hpp>
+
+#if !defined(BOOST_PFR_INTERFACE_UNIT)
+#include <type_traits>
+#endif
+
+namespace boost { namespace pfr { namespace detail {
+
+// This function serves as a link-time assert. If linker requires it, then
+// `unsafe_declval()` is used at runtime.
+void report_if_you_see_link_error_with_this_function() noexcept;
+
+// For returning non default constructible types. Do NOT use at runtime!
+//
+// GCCs std::declval may not be used in potentionally evaluated contexts,
+// so we reinvent it.
+template <class T>
+constexpr T unsafe_declval() noexcept {
+    report_if_you_see_link_error_with_this_function();
+
+#ifdef BOOST_PFR_USE_LEGACY_UNSAFE_DECLVAL_IMPLEMENTATION
+    typename std::remove_reference<T>::type* ptr = nullptr;
+    ptr += 42; // suppresses 'null pointer dereference' warnings
+    return static_cast<T>(*ptr);
+#else
+    // Looks like `static_cast<T>(*ptr)` to prvalue fails on clang in C++26.
+    // If this new implementation does not work for some cases, please, fill a
+    // bug report and feel free to
+    // define BOOST_PFR_USE_LEGACY_UNSAFE_DECLVAL_IMPLEMENTATION.
+    using func_ptr_t = T(*)();
+    func_ptr_t ptr = nullptr;
+    return ptr();
+#endif
+}
+
+}}} // namespace boost::pfr::detail
+
+
+#endif // BOOST_PFR_DETAIL_UNSAFE_DECLVAL_HPP
+