use fixed_string to create named arg class with static name for _a literal

include/fmt/core.h

@@ -979,16 +979,22 @@ struct arg_data<T, Char, NUM_ARGS, 0> {
 template <typename Char>
 inline void init_named_args(named_arg_info<Char>*, int, int) {}
 
-template <typename Char, typename T, typename... Tail>
+template <typename T> struct is_named_arg : std::false_type {};
+
+template <typename T, typename Char>
+struct is_named_arg<named_arg<Char, T>> : std::true_type {};
+
+template <typename Char, typename T, typename... Tail,
+          FMT_ENABLE_IF(!is_named_arg<T>::value)>
 void init_named_args(named_arg_info<Char>* named_args, int arg_count,
                      int named_arg_count, const T&, const Tail&... args) {
   init_named_args(named_args, arg_count + 1, named_arg_count, args...);
 }
 
-template <typename Char, typename T, typename... Tail>
+template <typename Char, typename T, typename... Tail,
+          FMT_ENABLE_IF(is_named_arg<T>::value)>
 void init_named_args(named_arg_info<Char>* named_args, int arg_count,
-                     int named_arg_count, const named_arg<Char, T>& arg,
-                     const Tail&... args) {
+                     int named_arg_count, const T& arg, const Tail&... args) {
   named_args[named_arg_count++] = {arg.name, arg_count};
   init_named_args(named_args, arg_count + 1, named_arg_count, args...);
 }
@@ -997,11 +1003,6 @@ template <typename... Args>
 FMT_CONSTEXPR FMT_INLINE void init_named_args(std::nullptr_t, int, int,
                                               const Args&...) {}
 
-template <typename T> struct is_named_arg : std::false_type {};
-
-template <typename T, typename Char>
-struct is_named_arg<named_arg<Char, T>> : std::true_type {};
-
 template <bool B = false> constexpr size_t count() { return B ? 1 : 0; }
 template <bool B1, bool B2, bool... Tail> constexpr size_t count() {
   return (B1 ? 1 : 0) + count<B2, Tail...>();
@@ -1277,10 +1278,10 @@ template <typename Context> struct arg_mapper {
     return val;
   }
 
-  template <typename T>
-  FMT_CONSTEXPR FMT_INLINE auto map(const named_arg<char_type, T>& val)
-      -> decltype(std::declval<arg_mapper>().map(val.value)) {
-    return map(val.value);
+  template <typename T, FMT_ENABLE_IF(is_named_arg<T>::value)>
+  FMT_CONSTEXPR FMT_INLINE auto map(const T& named_arg)
+      -> decltype(std::declval<arg_mapper>().map(named_arg.value)) {
+    return map(named_arg.value);
   }
 
   unformattable map(...) { return {}; }

include/fmt/format.h

@@ -3941,6 +3941,24 @@ template <typename Char> struct udl_formatter {
   }
 };
 
+#  if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
+struct statically_named_arg : view {
+  static constexpr auto name = Str.data;
+
+  const T& value;
+  statically_named_arg(const T& v) : value(v) {}
+};
+
+template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
+struct is_named_arg<statically_named_arg<T, Char, N, Str>> : std::true_type {};
+
+template <typename Char, size_t N, fixed_string<Char, N> Str> struct udl_arg {
+  template <typename T> auto operator=(T&& value) const {
+    return statically_named_arg<T, Char, N, Str>(std::forward<T>(value));
+  }
+};
+#  else
 template <typename Char> struct udl_arg {
   const Char* str;
 
@@ -3948,6 +3966,7 @@ template <typename Char> struct udl_arg {
     return {str, std::forward<T>(value)};
   }
 };
+#  endif
 }  // namespace detail
 FMT_MODULE_EXPORT_BEGIN
 
@@ -3981,12 +4000,21 @@ constexpr detail::udl_formatter<wchar_t> operator"" _format(const wchar_t* s,
     fmt::print("Elapsed time: {s:.2f} seconds", "s"_a=1.23);
   \endrst
  */
+#  if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+template <detail::fixed_string Str>
+constexpr detail::udl_arg<remove_cvref_t<decltype(Str.data[0])>,
+                          sizeof(Str.data) / sizeof(decltype(Str.data[0])), Str>
+operator""_a() {
+  return {};
+}
+#  else
 constexpr detail::udl_arg<char> operator"" _a(const char* s, size_t) {
   return {s};
 }
 constexpr detail::udl_arg<wchar_t> operator"" _a(const wchar_t* s, size_t) {
   return {s};
 }
+#  endif
 }  // namespace literals
 
 FMT_MODULE_EXPORT_END