https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88052
--- Comment #2 from Jerry DeLisle <jvdelisle at gcc dot gnu.org> ---
Well until now we were permitting this as an extension. We could choose to
allow it only under -std=legacy or throw a runtime error. Usually we don't like
to throw runtime errors for things if not absolutely necessary. As this is a
constraint, we can give the error with F2008 or above.
In format.c we have:
default:
/* Assume a missing comma, this is a GNU extension */
goto format_item_1;
