That sort of thing is very dangerous with modern compilers.  Does K&R C require that variables be allocated in the order that they are declared?  If not, you're playing with fire.  To get decent performance out of modern processors, the compiler must perform data placement to maximize cache efficiency, and that practically guarantees that you can't rely on out-of-bounds array references.

Unless "foo" were a pointer that the programmer explicitly pointed to the inside of a larger data structure.  In that case you could guarantee that the construct would work reliably.  But by pointing into the middle of another data structure you've created a data aliasing situation, and that complicates compiler data flow analysis and can block important optimizations.

Things were much simpler when V7 was written.

-Paul W.