This might seem obviously “yes” at first, but consider a method like foo.debugRepr() which outputs the string FOO and has documentation which says it is meant only to be used for logging / debugging. Then you make a new release of your library and want to update the debug representation to be **FOO**.
Based on the semantics of debugRepr() I would argue that this is NOT a breaking change even though it is returning a different value, because it should only affect logging. However, if someone relies on this and uses it the wrong way, it will break their code.
What do you think? Is this a breaking change or not?
No, it’s not a breaking change IMO. The method contract (the “debug” name, the comment) heavily implies the output may change and should not be relied upon.