[CamouflagedKiwi]

This doesn't sound like the kind of portability anyone is really worried about. I get that the docs on the linked site are written in standards-ese and are complicated by macro replacement, but I don't think the outcome of sending your credit card details away is gonna be an outcome. If it was, an uncharitable implementation with access to your card details would be free to do that any time you gave it input invoking undefined behaviour (which is of course not uncommon, especially in incorrect code).

[afiori]

which makes me consider an interesting distinction, undefined behavior refers to the behavior of the compiler output, does the C standard "allow" compilers to do compile-time code executions with undefined behavior? is the runtime behavior of the compiler even in scope for the standard in general?