That is not true and very important to understand. The optimizations a compiler does by eliding UB stuff are often very effective and I think no one in the thre

That is not true and very important to understand. The optimizations a compiler does by eliding UB stuff are often very effective and I think no one in the thread has questioned that. It is a fallacy, though, to think that these optimizations could be replaced by diagnostics. That would be hard in most cases and sometimes impossible.