我有一段代码依赖于某个case类的任意元素的存在，以便在该类的字段上进行操作。这里有some options，尽管几乎每个Scala博客都建议不要使用null，但在类型级编程中似乎并不可怕（例如，以下答案：{ {3}}）。但是，这段直接将null强制转换为特定案例类的代码不起作用，并且不会引发错误，我对原因很感兴趣。

trait V {}
case class W(x: Int) extends V

val y = null.asInstanceOf[W]

y match {
  case w:W => println("null cast to W is a W")
  case _ => println("null cast to W is NOT a W")
}

// prints "null cast to W is NOT a W"

y match {
  case v:V => println("null cast to W is a V")
  case _ => println("null cast to W is NOT a V")
}

// prints "null cast to W is NOT a V"

val z = W(1)

z match {
  case w:W => println("instantiated W is a W")
  case _ =>  println("instantiated W is NOT a W")
  }

// prints "instantiated W is a W"

z match {
  case v:V =>  println("instantiated W is a V")
  case _ =>  println("instantiated W is NOT a V")
}

// prints "instantiated W is a V"