Well, but the traditional preprocessor behavior in the end is not in not
expanding macro arguments, normally they actually are expanded, but not
immediately, but because of trying to preprocess the result of preprocessing
again and again.
When traditionally preprocessing #if/#elif etc. directives, the directive
line is preprocessed until no macros are present and only then fed to the
ISO preprocessor to evaluate and handle the directive.
The problem with the builtin function-like macros is that it needs the
argument immediately.