@jensmaurer: so you're sure that there are cases left now where arrays of unknown bound aren't allowed?

Hm... Maybe the phrasing is still too vague to properly handle

void f(C<sizeof(T[])> x);

(This isn't allowed, because we're applying sizeof to an incomplete type, but the phrasing "in a function parameter" might suggest it's giving permission to write something like that.)

Good find. OK, let's think about this some more.