Yes, it's in the article. > I don't see how [. . .] is undefined behaviour. This is a perfectly meaningless sentence. "Undefined behavior" doesn't mean "thing

Yes, it's in the article. > I don't see how [. . .] is undefined behaviour. This is a perfectly meaningless sentence. "Undefined behavior" doesn't mean "things which don't make sense in a particular implementation", it literally means behavior not defined by the spec. The spec declines to define the meaning of p==q when p and/or q is a freed pointer. There's nothing to "see" here: it's just not defined.