Scala自定义继承Set的集合：SetOps类型参数修改遇阻

这是此前「如何在Scala中创建继承Set的自定义集合」问题的跟进。我已经实现了一个继承自mutable.Set的基础类TestSet，filter、collect等多数方法都能正常运行（代码如下）。原本打算将SetOps的类型参数从mutable.SetOps[A, mutable.Set, TestSet[A]]修改为mutable.SetOps[A, TestSet, TestSet[A]]，但在重定义iterableFactory时遇到了困难。

可运行代码片段：

import scala.collection._

class TestSet[A]
  extends mutable.Set[A]
    with mutable.SetOps[A, mutable.Set, TestSet[A]] {

  private var data = mutable.Set.empty[A]

  override protected def fromSpecific(coll: IterableOnce[A]): TestSet[A] = TestSet.from(coll)

  override protected def newSpecificBuilder: mutable.Builder[A, TestSet[A]] = TestSet.newBuilder

  override def empty: TestSet[A] = TestSet.empty

  override def clear(): Unit = {
    data = mutable.Set.empty
  }

  override def subtractOne(elem: A): TestSet.this.type = {
    data -= elem
    this
  }

  override def addOne(elem: A): TestSet.this.type = {
    data += elem
    this
  }

  override def contains(elem: A): Boolean = data.contains(elem)

  override def iterator: Iterator[A] = {
    data.iterator
  }

}

object TestSet {
  def empty[A] = new TestSet[A]


  def from[A](source: IterableOnce[A]): TestSet[A] =
    source match {
      case pm: TestSet[A] => pm
      case _ => (newBuilder ++= source).result()
    }

  def apply[A](elem: A*): TestSet[A] = from(elem)

  def newBuilder[A]: mutable.Builder[A, TestSet[A]] =
    new mutable.GrowableBuilder[A, TestSet[A]](empty)

  import scala.language.implicitConversions

  implicit def toFactory[A](self: this.type): Factory[A, TestSet[A]] =
    new Factory[A, TestSet[A]] {
      def fromSpecific(it: IterableOnce[A]): TestSet[A] = self.from(it)

      def newBuilder: mutable.Builder[A, TestSet[A]] = self.newBuilder
    }

}

内容的提问来源于stack exchange，提问作者Usr654789