I assume xyzzyz was referring to the undecidability of the halting problem, not just user input. Comparisons to the halting problem are often made inappropriat

I assume xyzzyz was referring to the undecidability of the halting problem, not just user input. Comparisons to the halting problem are often made inappropriately, as would be the case here, since the compiler doesn't need to answer whether an arbitrary program contains aliasing, but only identify programs that admit arguments (often simple ones) demonstrating that they don't alias. It's still undecidable, though.