Moved boost::swap into a detail namespace to avoid it being found via…

… ADL.

This resolves infinite recursion when boost::swap is called on types that
are defined in namespace boost. The swap_impl implementation and noexcept
specification is using an unqualified call to swap, so if the argument types
are defined in namespace boost then our boost::swap function may be found
and selected to resolve the call.

Fixes #148.

include/boost/core/swap.hpp

@@ -73,16 +73,27 @@ namespace boost_swap_impl
 }
 
 namespace boost
+{
+namespace swap_detail
 {
   template<class T1, class T2>
   BOOST_GPU_ENABLED
-  typename enable_if_c< !boost_swap_impl::is_const<T1>::value && !boost_swap_impl::is_const<T2>::value >::type
+  typename enable_if_c< !::boost_swap_impl::is_const<T1>::value && !::boost_swap_impl::is_const<T2>::value >::type
   swap(T1& left, T2& right)
     BOOST_CORE_SWAP_NOEXCEPT_IF(BOOST_NOEXCEPT_EXPR(::boost_swap_impl::swap_impl(left, right)))
   {
     ::boost_swap_impl::swap_impl(left, right);
   }
-}
+} // namespace swap_detail
+
+// We need to import the swap name with a using directive here to prevent boost::swap from being found
+// via ADL for types defined in namespace boost. This prevents infinite recursion as we use unqualified
+// call to swap in noexcept specification and implementation of swap_impl.
+// Note: Users should always refer to the swap function as boost::swap, i.e. not unqualified swap
+// or boost::swap_detail::swap. The swap_detail namespace is an implementation detail of Boost.Swap.
+using namespace swap_detail;
+
+} // namespace boost
 
 #undef BOOST_CORE_SWAP_NOEXCEPT_IF
 

test/swap/Jamfile.v2

@@ -14,6 +14,7 @@ compile swap_lib_header_2.cpp ;
 compile swap_mixed_headers_1.cpp ;
 compile swap_mixed_headers_2.cpp ;
 compile swap_noexcept.cpp ;
+compile swap_boost_adl_recursion.cpp ;
 
 compile-fail swap_const_wrapper_fail.cpp ;
 

test/swap/swap_boost_adl_recursion.cpp

@@ -0,0 +1,40 @@
+// 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 with types in namespace boost is not
+// found via ADL and does not introduce infinite recursion.
+
+#include <boost/core/swap.hpp>
+
+namespace boost {
+
+struct boost_type {};
+
+}
+
+namespace test_ns {
+
+template<class T> class X
+{
+    X( X const& );
+
+public:
+
+    X() {}
+};
+
+template< typename T1, typename T2 >
+inline void swap(T1&, T2&)
+{
+}
+
+}
+
+int main()
+{
+    test_ns::X< boost::boost_type > x;
+    return noexcept( boost::swap( x, x ) );
+}