Call parse on empty specs at compile time

test/format-test.cc

@@ -2101,7 +2101,7 @@ struct check_back_appender {};
 
 FMT_BEGIN_NAMESPACE
 template <> struct formatter<check_back_appender> {
-  auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
+  FMT_CONSTEXPR auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
     return ctx.begin();
   }