Use std::invoke_result in std::async instead of std::result_of

	* include/std/future (__async_result_of): Use __invoke_result instead
	of result_of.

From-SVN: r259888

libstdc++-v3/ChangeLog

 2018-05-03  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/future (__async_result_of): Use __invoke_result instead
+	of result_of.
+
 	* include/std/any (any_cast): Use __remove_cvref_t.
 	* include/std/tuple (__make_tuple): Likewise.
 	* include/std/type_traits (__remove_cvref_t): Define.

libstdc++-v3/include/std/future

@@ -181,8 +181,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // 2021. Further incorrect usages of result_of
   template<typename _Fn, typename... _Args>
-    using __async_result_of = typename result_of<
-      typename decay<_Fn>::type(typename decay<_Args>::type...)>::type;
+    using __async_result_of = typename __invoke_result<
+      typename decay<_Fn>::type, typename decay<_Args>::type...>::type;
 
   template<typename _Fn, typename... _Args>
     future<__async_result_of<_Fn, _Args...>>