https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98142
--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> --- It probably makes sense to use the Clang __attribute__((enum_extensibility(closed)) spelling of this attribute, even though I don't see the distinction between "open" and no attribute. https://clang.llvm.org/docs/AttributeReference.html#enum-extensibility
