Not so sealed

Foo.scala

sealed trait Foo
case class Bar(x: Int) extends Foo
case class Baz(name: String) extends Foo

object Foo {
  def show(f: Foo) =
    f match {
      case Bar(x) => x.toString // Quux, a subtype of Bar in a different file, will be matched as a Bar
      case Baz(name) => name
    }
}

Quux.scala

// Per http://www.scala-lang.org/files/archive/spec/2.11/05-classes-and-objects.html#sealed, we can extend
// members of the sealed hierarchy in separate units.
case class Quux(x: Int) extends Bar(x)

object Test extends App {
  println(Foo.show(Quux(2)))
}