./tests/patmat/t6420.scala:5: warning: match may not be exhaustive.
It would fail on the following input: (Nil, _), (List(_, _), _), (Nil, Nil), (Nil, List(_, _)), (List(_, _), Nil), (List(_, _), List(_, _)), (_, Nil), (_, List(_, _))
  def foo(x: List[Boolean], y: List[Boolean]) = (x,y) match {
                                                      ^
one warning found