EDIT: This comment is wrong, see fsmv’s comment below. Leaving for posterity because I’m no coward!

- - -

Undefined behavior only means that the spec leaves a particular situation undefined and that the compiler implementor can do whatever they want. Every compiler defines undefined behavior, whether it’s documented (or easy to qualify, or deterministic) or not.

It is in poor taste that gcc has had widely used, documented behaviors that are changing, especially in a point release.