I think that the reason people want this stuff is that they don't understand how much effort it takes to prove that underlying UB is not triggered in a branch a

I think that the reason people want this stuff is that they don't understand how much effort it takes to prove that underlying UB is not triggered in a branch and optimize out checks. Code size would explode, too; decimating the usefulness of embedded platforms, and eviscerating instruction caches. In that way, it would also be an ecological disaster.