I looked at the documentation of [LabelDef](https://github.com/scala/scala/blob/master/src/reflect/scala/reflect/api/Trees.scala#L747) and it says that it is only used for while-/do-while-loops and for some pattern matching things. After passing `LabelDef` is general the test suite still runs fine. Can you show a code example with such a tail call that uses `LabelDef` and must be highlighted?