Your first part was irrelevant: if you cast away const, of course variable must have been const in first place. As other have already mentioned, casting itself is not an undefined behavior (and sometimes even have legitimate reasons, mentioned in my earlier post). On the other hand, trying to modify const variable though non-const pointer is undefined behavior.

P.S. if you downvoted to disagree with this (on my parent post), please provide an opposite example, because as a C programmer, I would really be interested to be proven wrong on this matter.