Stuff.scala

import scala.language.higherKinds

sealed abstract class Stuff[+A]

case class Suspend[A](a: () => mutable.Set[A])
  extends Stuff[A]
                                      
def extract[A](source: Stuff[A]): Suspend[A] =
  source match {
    case ref @ Suspend(_) => ref
}

//<pastie>:25: error: type mismatch;
// found   : Suspend[?A1] where type ?A1 <: A (this is a GADT skolem)
// required: Suspend[A]
//Note: ?A1 <: A, but class Suspend is invariant in type A.
//You may wish to define A as +A instead. (SLS 4.5)
//           case ref @ Suspend(_) => ref
//                                    ^