title: Scala compilation error
body: <p>Can't figure out what's wrong with StrangeIntQueue extending Queue, why there is an error "Not enough arguments for constructor Queue: (leading: Int)list.Lister.Queue[Int]. Unspecified value parameter leading". How can I specify it?</p>
  
  <pre><code>  class Queue[+T](
      private val leading: T
    ) {
      def enqueue[U &gt;: T](x: U) =
        new Queue[U](leading: U) // ...
    }
  
    class StrangeIntQueue(private val leading: Int) extends Queue[Int] {
      override def enqueue(x: Int) = {
        println(math.sqrt(x))
        super.enqueue(x)
      }
    }
  </code></pre>