result_of -> invoke_result_t

include/fmt/core.h

@@ -215,18 +215,15 @@ using std_string_view = std::experimental::basic_string_view<Char>;
 template <typename T> struct std_string_view {};
 #endif
 
-template <typename> struct result_of;
-
 #if (__cplusplus >= 201703L ||                          \
      (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)) && \
     __cpp_lib_is_invocable >= 201703L
 template <typename F, typename... Args>
-struct result_of<F(Args...)> : std::invoke_result<F, Args...> {};
+using invoke_result_t = std::invoke_result_t<F, Args...>;
 #else
-// A workaround for gcc 4.4 that doesn't allow F to be a reference.
+// An implementation of invoke_result for pre-C++17.
 template <typename F, typename... Args>
-struct result_of<F(Args...)>
-    : std::result_of<typename std::remove_reference<F>::type(Args...)> {};
+using invoke_result_t = typename std::result_of<F(Args...)>::type;
 #endif
 
 // Casts nonnegative integer to unsigned.
@@ -906,8 +903,8 @@ template <typename Context> class basic_format_arg {
       const T& value);
 
   template <typename Visitor, typename Ctx>
-  friend FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type
-  visit_format_arg(Visitor&& vis, const basic_format_arg<Ctx>& arg);
+  friend FMT_CONSTEXPR internal::invoke_result_t<Visitor, int> visit_format_arg(
+      Visitor&& vis, const basic_format_arg<Ctx>& arg);
 
   friend class basic_format_args<Context>;
   friend class internal::arg_map<Context>;
@@ -949,7 +946,7 @@ struct monostate {};
   \endrst
  */
 template <typename Visitor, typename Context>
-FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type visit_format_arg(
+FMT_CONSTEXPR internal::invoke_result_t<Visitor, int> visit_format_arg(
     Visitor&& vis, const basic_format_arg<Context>& arg) {
   typedef typename Context::char_type char_type;
   switch (arg.type_) {
@@ -988,8 +985,8 @@ FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type visit_format_arg(
 }
 
 template <typename Visitor, typename Context>
-FMT_DEPRECATED FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type
-visit(Visitor&& vis, const basic_format_arg<Context>& arg) {
+FMT_DEPRECATED FMT_CONSTEXPR internal::invoke_result_t<Visitor, int> visit(
+    Visitor&& vis, const basic_format_arg<Context>& arg) {
   return visit_format_arg(std::forward<Visitor>(vis), arg);
 }