That's a good point. I doubt most languages generating C put a lot of thought into what subset is supported in proprietary compilers. If anyone tries one, then there could be profiles done for unusual compilers where the generated code has certain properties. Far as the past, I think people couldve tested the output just to see what happened. Then, either ditch high-level language per fuction, module, or project depending on how far incompatibility effects went.

How does that sound?