Perhaps in c-with-classes(Cpre)? To the extent that its output could be considered C. It looks like you're right and the answer to when was C++ a superset of C

Perhaps in c-with-classes(Cpre)? To the extent that its output could be considered C. It looks like you're right and the answer to when was C++ a superset of C may well be "never". From the description, Cfront had always been a full-fledged parser that only happened to output C since the very beginning.