Qualify std::__invoke in <variant> to prevent ADL

	* include/std/variant (__gen_vtable_impl::__visit_invoke): Qualify
	__invoke to prevent ADL.

From-SVN: r260254

libstdc++-v3/ChangeLog

+2018-05-15  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/variant (__gen_vtable_impl::__visit_invoke): Qualify
+	__invoke to prevent ADL.
+
 2018-05-14  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/81256

libstdc++-v3/include/std/variant

@@ -838,9 +838,8 @@ namespace __variant
       decltype(auto)
       static constexpr __visit_invoke(_Visitor&& __visitor, _Variants... __vars)
       {
-	return __invoke(std::forward<_Visitor>(__visitor),
-			std::get<__indices>(
-			    std::forward<_Variants>(__vars))...);
+	return std::__invoke(std::forward<_Visitor>(__visitor),
+	    std::get<__indices>(std::forward<_Variants>(__vars))...);
       }
 
       static constexpr auto