./tests/patmat/t9351.scala:8: warning: match may not be exhaustive.
It would fail on the following input: _: A
    a match {
      ^
./tests/patmat/t9351.scala:17: warning: match may not be exhaustive.
It would fail on the following input: (_, _), (_, None), (_, Some(_))
    (a, o) match {
           ^
./tests/patmat/t9351.scala:28: warning: match may not be exhaustive.
It would fail on the following input: (_, _)
    (a, b) match {
           ^
three warnings found