The parent said > UB problems happen only when compilers detect it, understand it is an opportunity for optimization, because they are allowed to do anything,

The parent said > UB problems happen only when compilers detect it, understand it is an opportunity for optimization, because they are allowed to do anything, then do that optimization. Perhaps I'm misunderstanding the comment, but I took "detect it, understand it is an opportunity for optimization, because they are allowed to do anything, ..." to mean "detect that undefined behavior occurs and then ..." Instead, it should be "detect that an optimization can be applied and that the optimized code will do the right thing in all cases that undefined behavior does not occur, and apply the optimization." I've met programmers who seem to believe that compilers intentionally apply weird transformations when they detect undefined behavior. But really, the compilers are applying sensible transformations for well-defined code and ignoring whether those transformations are sensible for code that isn't well-defined.