Replace 'std::result_of' by 'std::invoke_result' where possible

C++17 deprecated 'std::result_of' in favour of 'std::invoke_result' and will ban it outright in C++20.

- Replace all uses of 'internal::result_of' in {fmt} by 'internal::invoke_result'.
- Import name 'std::invoke_result' into fmt's internal namespace when compiling C++17 mode.
- Implement 'internal::invoke_result' in terms of 'std::result_of' when compiling in modes C++11 or C++14.

Signed-off-by: Daniela Engert <[email protected]>

include/fmt/core.h

@@ -215,13 +215,17 @@ namespace internal {
 template <typename T>
 typename std::add_rvalue_reference<T>::type declval() FMT_NOEXCEPT;
 
-template <typename> struct result_of;
-
-template <typename F, typename... Args> struct result_of<F(Args...)> {
-  // A workaround for gcc 4.4 that doesn't allow F to be a reference.
-  typedef typename std::result_of<typename std::remove_reference<F>::type(
-      Args...)>::type type;
-};
+#if (__cplusplus >= 201703L ||                          \
+     (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)) && \
+    __cpp_lib_is_invocable >= 201703L
+using std::invoke_result;
+#else
+template <typename> struct invoke_result;
+// A workaround for gcc 4.4 that doesn't allow F to be a reference.
+template <typename F, typename... Args>
+struct invoke_result<F(Args...)>
+    : std::result_of<typename std::remove_reference<F>::type(Args...)> {};
+#endif
 
 // Casts nonnegative integer to unsigned.
 template <typename Int>
@@ -847,7 +851,7 @@ 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
+  friend FMT_CONSTEXPR typename internal::invoke_result<Visitor(int)>::type
   visit_format_arg(Visitor&& vis, const basic_format_arg<Ctx>& arg);
 
   friend class basic_format_args<Context>;
@@ -888,7 +892,7 @@ struct monostate {};
   \endrst
  */
 template <typename Visitor, typename Context>
-FMT_CONSTEXPR typename internal::result_of<Visitor(int)>::type visit_format_arg(
+FMT_CONSTEXPR typename internal::invoke_result<Visitor(int)>::type visit_format_arg(
     Visitor&& vis, const basic_format_arg<Context>& arg) {
   typedef typename Context::char_type char_type;
   switch (arg.type_) {
@@ -927,7 +931,7 @@ 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
+FMT_DEPRECATED FMT_CONSTEXPR typename internal::invoke_result<Visitor(int)>::type
 visit(Visitor&& vis, const basic_format_arg<Context>& arg) {
   return visit_format_arg(std::forward<Visitor>(vis), arg);
 }