Test error on invalid type spec and remove unused alias

include/fmt/format.h

@@ -3773,8 +3773,6 @@ struct formatter<
   }
 
  private:
-  using arg_ref = internal::arg_ref<Char>;
-
   internal::dynamic_format_specs<Char> specs_;
 };
 

test/format-test.cc

@@ -1909,4 +1909,5 @@ TEST(FormatTest, FormatStringErrors) {
   EXPECT_ERROR("{}{1}",
                "cannot switch from automatic to manual argument indexing",
                int, int);
+  EXPECT_ERROR("{:s}", "invalid type specifier", int);
 }