Add boost::core::launder

test/launder_test.cpp

@@ -0,0 +1,34 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/core/launder.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <new>
+
+struct X
+{
+    int v_;
+
+    explicit X( int v = 0 ): v_( v ) {}
+};
+
+int main()
+{
+    X x;
+
+    typedef X const CX;
+
+    ::new( &x ) CX( 1 );
+    X const* px1 = &x;
+
+    BOOST_TEST_EQ( px1->v_, 1 );
+
+    ::new( &x ) CX( 2 );
+    X const* px2 = boost::core::launder( px1 );
+
+    BOOST_TEST_EQ( px1, px2 );
+    BOOST_TEST_EQ( px2->v_, 2 );
+
+    return boost::report_errors();
+}