scala – 如何使用无类型约束来特化特征？

我正在尝试创建一个特征层次结构,它基本上描述了图形中的顶点 – 任何可以有父列表和子列表的对象.

进一步来说：

> 1)分层可能有也可能没有父母.
> 2)如果是,则父母是分层的.
> 3)Hierarchical将有子级,也是Hierarchical.

这是我的一般特征：

/*
  Hierarchical takes two type parameters: Parent and Child. 
  These should be Hierarchicals with parameters Parent and Child as well.
*/
trait Hierarchical[Parent <: Hierarchical[Parent,Child],Child <: Hierarchical[Parent,Child]] {

  // parents must be passed by constructor lest they are treated as None. 
  // (Requirement 1,2)
  val parents: Option[List[Hierarchical[Parent,Child]]] = None

  // children can be added,so mutability needed (Requirement 3)
  var children: List[Hierarchical[Parent,Child]] = List()

  def addChild(child: Child) = children ++= List(child)
}

对此的一个特殊化是一个“根”顶点,它的父节点不存在,所以根本不应该指定类型 – 因此,它是None.这是我尝试专门化Hierarchical特征：

/*
  A RootHierarchical has no parents. It does,however,have
  children whose parents are RootHierarchicals and whose children
  are Child.
  A parent can be none (Requirement 1)
*/
trait RootHierarchical[Child <: Hierarchical[RootHierarchical,Child]]
  extends Hierarchical[None.type,Child] {
  override val parents = None
}

IntelliJ对此很满意,但我写的测试不是编译：

import org.scalatest.FlatSpec

class TestHierarchy extends FlatSpec {

  "A Hierarchical object" should "be constructable" in {

    /*
    Create a dummy
     */
    class DummyHierarchical(override val parents: List[DummyParentHierarchical])
      extends Hierarchical[DummyParentHierarchical,DummyHierarchical]

    class DummyParentHierarchical extends RootHierarchical[DummyHierarchical]

    val dummyParent = new DummyParentHierarchical
    val dummyChild = new DummyHierarchical(List(dummyParent))
    dummyParent.addChild(dummyChild)

    assert(dummyParent.parents.isEmpty)
    assert(dummyParent.children.nonEmpty)
  }
}

错误在这里：

Error:(14,11) type arguments [None.type,Child] do not conform to trait Hierarchical's type parameter bounds [Parent <: Hierarchical[Parent,Child] {

我的问题是：我怎样才能创建这种专业化？我知道None.type不是Hierarchical类型,但我怎么能专门针对它呢？