These optimization opportunities aren't small gains. They have big consequences, for example when they cause code that would not be vectorized to be vectorized. Again, compiler authors don't add UB optimization for the fun of it. Patches to add theoretical optimizations that don't actually move the needle are routinely rejected from LLVM and GCC (as they should, because optimizations slow down compilation, so they need to pull their weight). Rather, they add UB optimization when code is shown to benefit, often the code that people come to their bug trackers with complaining that it doesn't optimize to what they expect.
If there were a simple and reliable way to say "make the program crash if we hit any of this UB, rather than optimising it completely away" I think that would make a lot of people happy.
Is there a way to use that to address the "NeverCalled" example?
I feel like there's a huge disconnect here. Even after the strange behavior is explained, some people say "wow, I never ever want that behavior, how do I reliably avoid it?" but others respond "there's no problem, you're just using it wrong".
