> Are modern C compilers actually still single pass?

All except ImportC, which effortlessly handles forward references. (Mainly because ImportC hijacks the D front end to do the semantics.)

A bit of an aside but I was poking around in the SPIR-V spec yesterday and they can do forward references because the call site contains all the information to determine the function parameter types. Just thought it was interesting and not really something I had thought about before.