Comment by dataflow

> The only real counter-argument I could see is "fflush might terminate the program, hence we need to run the function before we know if UB will be triggered".

The thing to realize is there is no such thing as "UB will be triggered". The only thing that exists is "UB is triggered", combined with the as-if rule, which allows modifications that don't affect what the standard considers observable behavior. Or in other words, the standard defines a program according to its observable behavior. People think it's time-travel because they think of the program in terms of expressions and statements rather than side effects, but if you think of the programs in terms of observable behaviors rather than the lines of code executing, you see that there's no time travel.