is_described_class should be false for unions

include/boost/container_hash/is_described_class.hpp

@@ -6,6 +6,7 @@
 #define BOOST_HASH_IS_DESCRIBED_CLASS_HPP_INCLUDED
 
 #include <boost/type_traits/integral_constant.hpp>
+#include <boost/type_traits/is_union.hpp>
 #include <boost/describe/bases.hpp>
 #include <boost/describe/members.hpp>
 
@@ -17,7 +18,9 @@ namespace container_hash
 #if defined(BOOST_DESCRIBE_CXX11)
 
 template<class T> struct is_described_class: boost::integral_constant<bool,
-    describe::has_describe_bases<T>::value && describe::has_describe_members<T>::value>
+    describe::has_describe_bases<T>::value &&
+    describe::has_describe_members<T>::value &&
+    !boost::is_union<T>::value>
 {
 };