From what you wrote, it sounds like Clang's stance is "I don't know if this *could* be undefined behavior, so I'm going to take a massive dump on your executable, even though I'm not sure."
My question is then why don't other compilers do this? The answer is, because other compilers are sane. The sane thing to do is produce a warning saying "this line could be undefined behavior" and then try anyway, because it doesn't know if it's undefined behavior or not.