scala> def foo(x: Option[Int]) = x match { case None => }
1 warning found
-- [E029] Pattern Match Exhaustivity Warning: ----------------------------------
1 |def foo(x: Option[Int]) = x match { case None => }
  |                          ^
  |                          match may not be exhaustive.
  |
  |                          It would fail on pattern case: Some(_)
  |
  | longer explanation available when compiling with `-explain`
def foo(x: Option[Int]): Unit
