There's a guarantee relative to the address of b. But there's no guarantee about the relative addresses of a and b themselves, i.e. where they are placed in automatic storage. So even setting aside the idea of optimizing the test away at compile time, there's no guarantee that the comparison result will be a specific value. a could be above or below b, and they are not necessarily adjacent objects.
> If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined.
So although most implementations might produce an address that's relative to b, that's not actually guaranteed by the spec unless it's `&b + 1`.
... which it is in the program under discussion. Again, the address which has no guarantee relative to the address of b or the value of q is the address of a.
