Optimize format string parsing

test/custom-formatter-test.cc

@@ -24,7 +24,8 @@ class custom_arg_formatter
 
   custom_arg_formatter(fmt::format_context& ctx,
                        fmt::format_parse_context* parse_ctx,
-                       fmt::format_specs* s = nullptr)
+                       fmt::format_specs* s = nullptr,
+                       const char* = nullptr)
       : base(ctx, parse_ctx, s) {}
 
   using base::operator();

test/format-test.cc

@@ -1821,7 +1821,7 @@ class mock_arg_formatter
   using base = fmt::detail::arg_formatter_base<buffer_iterator, char>;
 
   mock_arg_formatter(fmt::format_context& ctx, fmt::format_parse_context*,
-                     fmt::format_specs* s = nullptr)
+                     fmt::format_specs* s = nullptr, const char* = nullptr)
       : base(ctx.out(), s, ctx.locale()) {
     EXPECT_CALL(*this, call(42));
   }