Disable boost::swap for const objects. Fixes #43.

include/boost/core/swap.hpp

@@ -21,13 +21,19 @@
 // avoid ambiguity when swapping objects of a Boost type that does
 // not have its own boost::swap overload.
 
+#include <boost/core/enable_if.hpp>
+#include <boost/config.hpp>
 #include <utility> //for std::swap (C++11)
 #include <algorithm> //for std::swap (C++98)
 #include <cstddef> //for std::size_t
-#include <boost/config.hpp>
 
 namespace boost_swap_impl
 {
+  // we can't use type_traits here
+
+  template<class T> struct is_const { enum _vt { value = 0 }; };
+  template<class T> struct is_const<T const> { enum _vt { value = 1 }; };
+
   template<class T>
   BOOST_GPU_ENABLED
   void swap_impl(T& left, T& right)
@@ -51,7 +57,8 @@ namespace boost
 {
   template<class T1, class T2>
   BOOST_GPU_ENABLED
-  void swap(T1& left, T2& right)
+  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_swap_impl::swap_impl(left, right);
   }

test/swap/Jamfile.v2

@@ -16,6 +16,9 @@ local compile_tests =
     mixed_headers_2.cpp
     ;
 
+local compile_fail_tests =
+    const_wrapper_fail.cpp ;
+
 local run_tests =
     primitive.cpp
     specialized_in_boost.cpp
@@ -49,6 +52,12 @@ rule test_all
         all_rules += [ compile $(file) : : "swap-$(test_name)" ] ;
     }
 
+    for file in $(compile_fail_tests)
+    {
+        local test_name = [ MATCH "([^.]*).cpp$" : $(file) ] ;
+        all_rules += [ compile-fail $(file) : : "swap-$(test_name)" ] ;
+    }
+
     for file in $(run_tests)
     {
         local test_name = [ MATCH "([^.]*).cpp$" : $(file) ] ;

test/swap/const_wrapper_fail.cpp

@@ -0,0 +1,28 @@
+// Copyright 2018 Andrzej Krzemieński
+// Copyright 2018 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+
+#include <boost/core/swap.hpp>
+
+namespace boost
+{
+  
+template<class T> struct Wrapper
+{
+    T value;
+};
+
+template<class T> inline void swap( Wrapper<T> & w, Wrapper<T> & v )
+{
+    boost::swap( w, v );
+}
+  
+} // namespace boost
+ 
+int main()
+{
+    boost::Wrapper<int> const w = { 2 };
+    boost::Wrapper<int> const v = { 3 };
+
+    swap( w, v );
+}