If the array index is unconditional, then sure that seems like a reasonable undefined behavior? If the access is conditional and the compiler can't infer whether the branch is taken, though, then it shouldn't make that assumption.

This reminds me of a bug in the msp430 variant of gcc. The compiler would replace "x <= literal" with "x < literal + 1" because it generated more efficient code. If the literal was UINT_MAX, though, the compiler would trudge ahead and roll over the literal to 0. It would then subsequently reason that the comparison was unconditionally false, and then completely optimize away the nominal path of my code. That was a frustrating day of debugging!