[lofenfew]

"Can't happen" comes from the fact that the compiler optimizes for defined behaviour, not undefined behaviour. Making defined behaviour fast at the expense of undefined behaviour looks the same as assuming undefined behaviour can't happen in a number of cases. Eg. if you dereference a pointer then check it for null, naturally the compiler will remove the later check. We can describe that as the compiler making the defined case fast, or we could describe it as the compiler assuming the pointer "can't" be null. The latter description is easy to think about, and generalizes broadly enough, to be a common way of describing it.