types.scala

trait Foo
class Ev[T]
case class Bar[A](xs: Ev[A], f: A => Unit) extends Foo

object Test {
  def main(args: Array[String]) {
    val foo: Foo = new Bar(new Ev[Int], println(_: Int))

    // This is disallowed.
    // foo match { case x: Bar[a] => x.f(x.xs) }
    // This isn't.
    foo match { case Bar(xs, f) => f(xs) }
  }
}
// java.lang.ClassCastException: Ev cannot be cast to java.lang.Integer
//   at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:105)
//   at Test$$anonfun$1.apply(a.scala:7)
//   at Test$.main(a.scala:12)