I am not so sure. Assuming the program does not do anything undefined is sort of the worst possible take on leaving the behavior undefined in the first place. I mean the behavior was left undefined so that "something" could be done. the language standard just does not know what that "something" is. Hell the compiler could do nothing and that would make more sense.

But to make optimizations pretending it is an invariant, it can't happen, when the specification clearly says it could happen. That's wild, and I would argue out of specification.