Pretty sure that means that tcc at least believes it is a C99 (or later) compiler; if it conformed to an earlier spec it (probably?) would have considered the inner `enum { C99 = 0 }` definition to be still in scope, and the return value would be 0.

I think.

That’s correct. I did check before posting that TCC 0.9.27 on Godbolt returned zero here, but I didn’t look beyond that. Evidently things have changed since that release.