Fix handling of exotic character types

include/fmt/core.h

@@ -1285,8 +1285,14 @@ template <typename Context> struct arg_mapper {
   FMT_CONSTEXPR FMT_INLINE auto map(T val) -> char_type {
     return val;
   }
-  template <typename T, FMT_ENABLE_IF(std::is_same<T, wchar_t>::value &&
-                                      !std::is_same<wchar_t, char_type>::value)>
+  template <typename T, enable_if_t<(std::is_same<T, wchar_t>::value ||
+#ifdef __cpp_char8_t
+                                     std::is_same<T, char8_t>::value ||
+#endif
+                                     std::is_same<T, char16_t>::value ||
+                                     std::is_same<T, char32_t>::value) &&
+                                        !std::is_same<T, char_type>::value,
+                                    int> = 0>
   FMT_CONSTEXPR FMT_INLINE auto map(T) -> unformattable_char {
     return {};
   }

test/core-test.cc

@@ -739,6 +739,11 @@ TEST(core_test, is_formattable) {
   static_assert(fmt::is_formattable<const signed char*>::value, "");
   static_assert(fmt::is_formattable<const unsigned char*>::value, "");
   static_assert(!fmt::is_formattable<wchar_t>::value, "");
+#ifdef __cpp_char8_t
+  static_assert(!fmt::is_formattable<char8_t>::value, "");
+#endif
+  static_assert(!fmt::is_formattable<char16_t>::value, "");
+  static_assert(!fmt::is_formattable<char32_t>::value, "");
   static_assert(!fmt::is_formattable<const wchar_t*>::value, "");
   static_assert(!fmt::is_formattable<const wchar_t[3]>::value, "");
   static_assert(!fmt::is_formattable<fmt::basic_string_view<wchar_t>>::value,