Since when does GCC *assume* the program to be correct?

Well said. Also, while in some cases it is a QoI issue which high-quality 
implementations will in some cases prescribe useful semantics for, this 
isn't such a case. I can't think of any particularly useful semantics for 
pointer wraparound, especially given that distinct objects have no defined 
nor stable relationships with each other anyway. Operating under the rules 
of modular arithmetic might have been nice, and perhaps a more recent 
language would define that...