Restore accidentally-removed volatile qualifier

	* include/std/functional (_Mu<_Arg, false, false>::operator()): Restore
	accidentally-removed volatile qualifier.

From-SVN: r231660

libstdc++-v3/ChangeLog

 2015-12-15  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/functional (_Mu<_Arg, false, false>::operator()): Restore
+	accidentally-removed volatile qualifier.
+
 	* include/std/functional (is_placeholder, is_bind_expression): Update
 	comments.
 	(_Safe_tuple_element): Replace with _Safe_tuple_element_t alias

libstdc++-v3/include/std/functional

@@ -852,7 +852,7 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
     public:
       template<typename _CVArg, typename _Tuple>
 	_CVArg&&
-	operator()(_CVArg&& __arg, _Tuple&) const
+	operator()(_CVArg&& __arg, _Tuple&) const volatile
 	{ return std::forward<_CVArg>(__arg); }
     };