Clean up argument mapping

include/fmt/format.h

@@ -3025,7 +3025,7 @@ class format_int {
 template <typename T, typename Char>
 struct formatter<T, Char,
                  enable_if_t<internal::type_constant<T, Char>::value !=
-                             internal::none_type>> {
+                             internal::custom_type>> {
   FMT_CONSTEXPR formatter() : format_str_(nullptr) {}
 
   // Parses format specifiers stopping either at the end of the range or at the

test/core-test.cc

@@ -229,8 +229,8 @@ struct custom_context {
 
 TEST(ArgTest, MakeValueWithCustomContext) {
   test_struct t;
-  fmt::internal::value<custom_context> arg =
-      fmt::internal::make_value<custom_context>(t);
+  fmt::internal::value<custom_context> arg(
+      fmt::internal::arg_mapper<custom_context>().map(t));
   custom_context ctx = {false, fmt::format_parse_context("")};
   arg.custom.format(&t, ctx.parse_context(), ctx);
   EXPECT_TRUE(ctx.called);
@@ -588,6 +588,10 @@ TEST(CoreTest, FormatForeignStrings) {
   EXPECT_EQ(fmt::format(my_string<wchar_t>(L"{}"), QString(L"42")), L"42");
 }
 
+struct implicitly_convertible_to_string {
+  operator std::string() const { return "foo"; }
+};
+
 struct implicitly_convertible_to_string_view {
   operator fmt::string_view() const { return "foo"; }
 };