I still don't get it. If it can't tell that the cmp_once is a constant expression why can it tell that cmp is a constant expression? EDIT: I think I get it. I

I still don't get it. If it can't tell that the cmp_once is a constant expression why can it tell that cmp is a constant expression? EDIT: I think I get it. In the event that the arguments are constant, something about the typeof business prevents it from being constant, so that's no good if required in a context where a constant is required. But, when not required to be a constant expression, you want to enforce single-evaluation of the arguments, so then cmp_once is used.