Propagate noexcept specification in boost::swap.

Mark boost::swap noexcept if the type supports non-throwing swap
implementation.

test/swap/Jamfile.v2

@@ -13,6 +13,7 @@ compile swap_lib_header_1.cpp ;
 compile swap_lib_header_2.cpp ;
 compile swap_mixed_headers_1.cpp ;
 compile swap_mixed_headers_2.cpp ;
+compile swap_noexcept.cpp ;
 
 compile-fail swap_const_wrapper_fail.cpp ;
 

test/swap/swap_noexcept.cpp

@@ -0,0 +1,42 @@
+// Copyright (c) 2023 Andrey Semashev
+//
+// 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)
+
+// Tests that boost::swap propagates noexcept specification correctly
+
+#include <boost/core/swap.hpp>
+#include <boost/config.hpp>
+
+#if !defined(BOOST_NO_CXX11_NOEXCEPT) && !defined(BOOST_NO_CXX11_STATIC_ASSERT) && \
+    !(defined(BOOST_GCC) && (BOOST_GCC < 40700))
+
+namespace test_ns {
+
+struct class_with_noexcept_swap
+{
+    static class_with_noexcept_swap& instance() noexcept;
+
+    friend void swap(class_with_noexcept_swap&, class_with_noexcept_swap&) noexcept
+    {
+    }
+};
+
+struct class_with_except_swap
+{
+    static class_with_except_swap& instance() noexcept;
+
+    friend void swap(class_with_except_swap&, class_with_except_swap&)
+    {
+    }
+};
+
+} // namespace test_ns
+
+static_assert(noexcept(boost::swap(test_ns::class_with_noexcept_swap::instance(), test_ns::class_with_noexcept_swap::instance())),
+    "boost::swap for class_with_noexcept_swap should have noexcept specification");
+static_assert(!noexcept(boost::swap(test_ns::class_with_except_swap::instance(), test_ns::class_with_except_swap::instance())),
+    "boost::swap for class_with_except_swap should not have noexcept specification");
+
+#endif // !defined(BOOST_NO_CXX11_NOEXCEPT) && !defined(BOOST_NO_CXX11_STATIC_ASSERT) ...