如何使scala类型有意挥发

我的用例如下：我想编写一个库,用A<：B公开类型A和B,B是易失性的.然后作为这个库的用户,我希望能够用类型B之一覆盖类型A的值：

trait TraitB {
  def doSomething: Unit
}

trait Library {
  type A
  type B <: A with TraitB
}

object LibraryUser {
  val library: Library = ???

  trait T {
    val x: library.A
  }

  object U extends T {
    val x: library.B = ???
  }
}

这失败并出现错误：

error: overriding value x in trait T of type LibraryUser.library.A;
value x has a volatile type; cannot override a member with non-volatile type

但是当我让A变得不稳定时,这就成功了.目前,我通过使它成为抽象类型Abstract的子类来实现A volatile,它在实现中只是Any,否则不使用：

trait TraitB {
  def doSomething: Unit
}

trait Library2 {
  protected type Abstract
  type A <: Any with Abstract
  type B <: A with TraitB
}

trait Library2ImplementationHelper {
  this: Library2 =>
  override type Abstract = Any
}

object Library2User {
  val library: Library2 = ???

  trait T {
    val x: library.A
  }

  object U extends T {
    val x: library.B = ???
  }
}

这编译并且工作正常,但抽象有点人为,我想知道是否还有其他方法.