Strictly C where your implementation has provided sane definitions of undefined behaviour is still C. The implementation is free to provide definitions of undef

Strictly C where your implementation has provided sane definitions of undefined behaviour is still C. The implementation is free to provide definitions of undefined behaviour, and no diagnostic required doesn't mean no diagnostic permitted. Offhand I think all the undetectable parts are a quirk of separate compilation which is solvable by linking an IR instead of machine code. That would probably be a high value project to implement.