scala前向引用扩展了定义

case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A,t: () => Stream[A]) extends Stream[A]

sealed trait Stream[+A] {

  def toList: List[A] = {
    val buf = new collection.mutable.ListBuffer[A]
    def go(s: Stream[A]): List[A] = s match {
      case Cons(h,t) =>
        buf += h()
        go(t())
      case _ => buf.toList
    }
    go(this)
  }

  def cons[A](hd: => A,tl: => Stream[A]): Stream[A] = Stream.cons(hd,tl)

  def empty = Stream.empty

  def unfold[A,S](z: S)(f: S => Option[(A,S)]): Stream[A] = f(z) match {
    case Some((h,t)) => cons(h,unfold(t)(f))
    case None => empty
  }

  def take(n: Int): Stream[A] = unfold((this,n)) {
    case (Cons(h,t),1) => Some((h(),(empty,0)))
    case (Cons(h,n) if n > 1 => Some((h(),(t(),n-1)))
    case (Empty,0) => None
  }
}

object Stream {

  def cons[A](hd: => A,tl: => Stream[A]): Stream[A] = Cons(() => hd,() => tl)

  def empty[A]: Stream[A] = Empty

  val ones: Stream[Int] = Stream.cons(1,ones)
}

object StreamTest {
  def main(args: Array[String]) {

    //why compile error: forward reference extends over definition of value ones
    /*val ones: Stream[Int] = Stream.cons(1,ones)
    println(ones.take(5).toList)*/

    println(Stream.ones.take(5).toList)
  }
}

为什么编译错误？：前向引用扩展了有价值的定义

在对象’Stream’中,
val ones：Stream [Int] = Stream.scons(1,ones)没问题

但在主要方法中,它不行(但是……同样的合成！)