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?
Breaking change. It’s gone from plain text to a markdown formatted text (possibly). There’s changing an interface (obviously a breaking change) and then there’s changing the semantics of a function. I just dealt with a breaking change where a string error value changed for an account registration api call. Previously it returned EMAIL_IN_USE and now it returns EMAIL_TAKEN. Same data type but it broke the client code. Changing values or formats is a breaking change. In your case the documentation says don’t rely on this function for anything but once the output is in the wild any monkey can start using it for anything and it can’t be certain that some code documentation will be consulted before deciding to depend on it.