[mrgriffin]

Would you expect IS_CONST to evaluate to the constant? With a name like that I would expect it to evaluate to true/false.

C here is asserting that the value inside is a constant and then evaluating to that constant.

[rurban]

Uh, you are right. C is the right name. It throws.

I mixed it up with a similar compile-time constness check in some of my libraries, where I decided if const of not. gcc throws, only clang could give me a proper answer.