Comment by stkdump

The problem with the preprocessor often is that it is a language inside another language and the preprocessor is designed to be almost completely agnostic of the language it is embedded in. So there might be subtle ways to use the preprocessor so that implementing as-if becomes very unintuitive. I don't have a good intuition about this case, if this is 100% designed in a way that it can never provoke such subtle side-effects. Basically what might end up happening is that the preprocessor has to learn some part of the C language to decide if such an as-if transformation is possible and then branch to either do it or don't.