Update test metafunction to use boost::declval instead of using null pointers

test/helpers/metafunctions.hpp

@@ -8,6 +8,7 @@
 
 #include <boost/config.hpp>
 #include <boost/type_traits/is_same.hpp>
+#include <boost/type_traits/declval.hpp>
 
 namespace test {
   template <class Container>
@@ -23,7 +24,9 @@ namespace test {
     BOOST_STATIC_CONSTANT(bool,
       value = sizeof(long) ==
               sizeof(flip(
-                ((Container*)0)->insert(*(typename Container::value_type*)0))));
+                (boost::declval<Container*>())
+                  ->insert(
+                    boost::declval<typename Container::value_type const&>()))));
   };
 }