The evidence in the form of CVEs suggests that we don't get it right.
> You mean the vector::operator[] is an example of fast-and-potentially-UB ? I don't see any UB if the constraints are met (keeping the index within valid range).
I don't understand this line of reasoning when it's plain that the contraints are not met. Routinely. It happens all the time. Again, see the CVEs.
Sure we all don't get it right 100% on the first write. For this reason we test and iterate. Equally you could say that it is pure hubris that a language will enable you to make your code right 100% of the time.
You mean the vector::operator[] is an example of fast-and-potentially-UB ? I don't see any UB if the constraints are met (keeping the index within valid range).
If they are not willing in a specific case they for sure had specific reasons not to be.