I can also explain, the pointer could be such that the pointer to field has no representation (i.e. overflows addressable memory). A pointer does not need to guarantee that an object of the size of its referent could be placed at its location.
(A bit more even, I think it is technically undefined behaviour if you offset a pointer that was constructed from a reference such that it points outside the region where the original one comes from–and I don't know if you are even allowed to have others. Pointers are allowed to track their underlying reference in llvm. Also, answer likely depends on resolution of the unsafe guidelines. This one in particular is interesting)