[flatfinger]

The ability to use pointers to structures with a Common initial Sequence goes back at least to 1974--before unions were invented. When C89 was written, it would have been plausible that an implementation could uphold the Common Initial Sequence guarantees for pointers without upholding them for unions, but rather less plausible that implementations could do the reverse. Thus, the Standard explicitly specified that the guarantee is usable for unions, but saw no need to redundantly specify that it also worked for pointers.

If compilers would recognize that operation involving a pointer/lvalue that is freshly visibly based on another is an action that at least potentially involves the latter, that would be sufficient to make code that relies upon the CIS work. Unfortunately, some compilers are willfully blind to such things.