Add boost::core::launder

include/boost/core/launder.hpp

@@ -0,0 +1,55 @@
+#ifndef BOOST_CORE_LAUNDER_HPP_INCLUDED
+#define BOOST_CORE_LAUNDER_HPP_INCLUDED
+
+// MS compatible compilers support #pragma once
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1020)
+# pragma once
+#endif
+
+//  Copyright 2023 Peter Dimov
+//  Distributed under the Boost Software License, Version 1.0.
+//  https://www.boost.org/LICENSE_1_0.txt
+
+#if defined(__has_builtin)
+# if __has_builtin(__builtin_launder)
+#  define BOOST_CORE_HAS_BUILTIN_LAUNDER
+# endif
+#endif
+
+#if !defined(BOOST_CORE_HAS_BUILTIN_LAUNDER)
+# include <new>
+#endif
+
+namespace boost
+{
+namespace core
+{
+
+#if defined(BOOST_CORE_HAS_BUILTIN_LAUNDER)
+
+template<class T> T* launder( T* p )
+{
+    return __builtin_launder( p );
+}
+
+#elif defined(__cpp_lib_launder)
+
+template<class T> T* launder( T* p )
+{
+    return std::launder( p );
+}
+
+#else
+
+template<class T> T* launder( T* p )
+{
+    return p;
+}
+
+#endif
+
+} // namespace core
+} // namespace boost
+
+#endif  // #ifndef BOOST_CORE_LAUNDER_HPP_INCLUDED