object Test {

  sealed abstract class ObjectValue {

    type ExternalRep
    def externalRep: ExternalRep

    override def equals(other: Any): Boolean = other match {
      case ObjectValue(otherExRep) => otherExRep == externalRep
      case _ => false
    }

  }

  object ObjectValue {
    def apply[ExRep](underlying: ExRep): ObjectValue = new ObjectValue {
      type ExternalRep = ExRep
      val externalRep = underlying
    }
    def unapply(ov: ObjectValue): Option[(ov.ExternalRep)] = Some(ov.externalRep)
  }
}