Not if they document it. But if you write a program that relies on those behaviors, you don't have C anymore, you have "GCC-C" or "MSVC-C".
I'm also not saying that's inherently a bad thing.
I'm also not saying that's inherently a bad thing.
I'm not sure this is helpful though. We've accepted "literally" also means "not literally" without creating a new language.
http://waste.typepad.com/waste/2014/07/on-a-popular-misconce...
It's just a figurative use of "literally". It's like saying "He's such a baby" of an immature (<- also figurative!) adult. You aren't saying that such an adult is actually a baby.