Add _a literal for named arguments

format.h

@@ -3030,6 +3030,16 @@ struct UdlFormat {
   }
 };
 
+template <typename Char>
+struct UdlArg {
+  const Char *str;
+
+  template <typename T>
+  NamedArg<Char> operator=(T &&value) const {
+    return {str, std::forward<T>(value)};
+  }
+};
+
 } // namespace internal
 
 inline namespace literals {
@@ -3039,6 +3049,11 @@ operator"" _format(const char *s, std::size_t) { return {s}; }
 inline internal::UdlFormat<wchar_t>
 operator"" _format(const wchar_t *s, std::size_t) { return {s}; }
 
+inline internal::UdlArg<char>
+operator"" _a(const char *s, std::size_t) { return {s}; }
+inline internal::UdlArg<wchar_t>
+operator"" _a(const wchar_t *s, std::size_t) { return {s}; }
+
 } // inline namespace literals
 } // namespace fmt
 #endif // FMT_USE_USER_DEFINED_LITERALS