It's very hard to be aware of the numerous details you find obvious yourself when you're communicating.

Also, by the way, I have an intuition that the issue might not specifically prevent the removal of a reverse operation. That is to say, if a series with some positives or negative integers does/doesn't overflow under a left fold, I have a hunch that it does/doesn't under a right fold.

If this can be proven, then that specific case is fine over integers (optimizing away reverse).

[1, intmax, -1]

So, yes, one direction overflows and one doesn't, but as long as the contract says overflow works as two's compliment, the answer is the same. This is why not having undefined behavior is so useful.

It doesn't have to be two's compliment - for any modular arithmetic, the answer will be the same.

But that's at a cost of giving up knowledge of whether it overflowed. In this case it overflows and underflows, when it does, an equal number of times, so there happens to be no problem. In some cases, all you care about is the answer modulo some number which jives well with your numbers' representation. In that case, there is also no problem.

In many cases, if there is a different number of overflows than underflows, you'd get the wrong answer without knowing it, and your answer is likely (but not guaranteed) to be very wrong when you do.

There are other approaches we can take. Overflow could saturate. In that case, we get the wrong answer in one direction here but it's only off by a little instead of a lot. Overflow could also trap, in which case we'd notice that we had risk of giving the wrong answer.

In either of these cases, we could not apply this optimization unless we allow for some measure of undefined behavior - which is why undefined behavior is so useful. It is absolutely the case that undefined behavior in a specification has significant drawbacks as well - but the issue at hand here is not the result of them.