covariant Queue Scala

class MyQueue[+T](private val rep: List[T] = Nil){

  def enqueue[S >: T](elem: S) =  new MyQueue[S](elem :: rep)

   def dequeue : MyQueue[T] = rep match{
      case _::t => new MyQueue[T](t)
      case Nil => throw new UnderflowException("Empty queue")
    }
     def isEmpty = rep == Nil

    def first: T = rep match{
      case h::t => h
      case Nil => throw new UnderflowException("Empty queue")
    }
  }

object MyQueue {
  def apply[T](xs: T*): MyQueue[T] = new MyQueue[T](xs.toList)

  def empty[T]: MyQueue[T] = new MyQueue[T](Nil)

}

object Tester{
  def main (args: Array[String]): Unit = {
    val queue = MyQueue.empty
    val queue1 = MyQueue()
    val queue2 = MyQueue('a', 'b', 'c')
    val queue3 = new MyQueue

    println(queue.enqueue(1).enqueue(2).dequeue.first)
    println(queue1.enqueue("first").enqueue("A").dequeue.first)
    println(queue2.dequeue.first)
    println(queue3.enqueue(1).enqueue(2).dequeue.first)
}
}