Simplify default formatter

include/fmt/format.h

@@ -3640,21 +3640,18 @@ FMT_CONSTEXPR auto write(OutputIt out, const T& value)
 // An argument visitor that formats the argument and writes it via the output
 // iterator. It's a class and not a generic lambda for compatibility with C++11.
 template <typename Char> struct default_arg_formatter {
-  using iterator = basic_appender<Char>;
   using context = buffered_context<Char>;
 
-  iterator out;
+  basic_appender<Char> out;
   basic_format_args<context> args;
-  locale_ref loc;
 
-  template <typename T> auto operator()(T value) -> iterator {
-    return write<Char>(out, value);
-  }
-  auto operator()(typename basic_format_arg<context>::handle h) -> iterator {
-    basic_format_parse_context<Char> parse_ctx({});
-    context format_ctx(out, args, loc);
+  void operator()(monostate) { report_error("argument not found"); }
+  template <typename T> void operator()(T value) { write<Char>(out, value); }
+  void operator()(typename basic_format_arg<context>::handle h) {
+    // Pass the default locale since the default output must be unlocalized.
+    auto parse_ctx = basic_format_parse_context<Char>({});
+    auto format_ctx = context(out, args, {});
     h.format(parse_ctx, format_ctx);
-    return format_ctx.out();
   }
 };
 
@@ -4125,12 +4122,8 @@ template <typename Char>
 void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
                 typename vformat_args<Char>::type args, locale_ref loc) {
   auto out = basic_appender<Char>(buf);
-  if (fmt.size() == 2 && equal2(fmt.data(), "{}")) {
-    auto arg = args.get(0);
-    if (!arg) report_error("argument not found");
-    arg.visit(default_arg_formatter<Char>{out, args, loc});
-    return;
-  }
+  if (fmt.size() == 2 && equal2(fmt.data(), "{}"))
+    return args.get(0).visit(default_arg_formatter<Char>{out, args});
 
   struct format_handler {
     basic_format_parse_context<Char> parse_context;
@@ -4160,9 +4153,8 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
     }
 
     FMT_INLINE void on_replacement_field(int id, const Char*) {
-      auto arg = get_arg(context, id);
-      context.advance_to(arg.visit(default_arg_formatter<Char>{
-          context.out(), context.args(), context.locale()}));
+      context.arg(id).visit(
+          default_arg_formatter<Char>{context.out(), context.args()});
     }
 
     auto on_format_specs(int id, const Char* begin, const Char* end)
@@ -4215,9 +4207,9 @@ FMT_CONSTEXPR FMT_INLINE auto native_formatter<T, Char, TYPE>::format(
       specs_.precision_ref.kind == arg_id_kind::none) {
     return write<Char>(ctx.out(), val, specs_, ctx.locale());
   }
-  auto specs = specs_;
-  handle_dynamic_spec(specs.width, specs.width_ref, ctx);
-  handle_dynamic_spec(specs.precision, specs.precision_ref, ctx);
+  auto specs = format_specs(specs_);
+  handle_dynamic_spec(specs.width, specs_.width_ref, ctx);
+  handle_dynamic_spec(specs.precision, specs_.precision_ref, ctx);
   return write<Char>(ctx.out(), val, specs, ctx.locale());
 }