Comment by eru

> Having implementation defined behavior would imply non-portability.

Not any more than UB does. Ie if you avoid the implementation defined behaviour, your code is perfectly portable.

(And otherwise your code could probe for what the implementation defined behaviour is, and adjust accordingly. No such luck with UB.)

However I do agree with you that it's hard to remove UB from the spec. The spec's failing is in having so much UB in the first place.