This is a bit of a weird one because it's a combination of the aliasing code and mixing C prototypes with QuakeC function definitions, and the function type rebuilding in qc-parse.y not being very "consistent" in its abuse of the type system.
It was even recent enough to compile first try.
It turns out that the optimizer completely breaks (a + b) % (a - b) with v6 code.
I don't remember what half these tests were for :/