Scala volatile 类型 : How is @uncheckedStable unsafe?

Question

我知道 Scala 中的 volatile 类型是用来建模的

the possibility that a type parameter or abstract type instance of a type does not have any non-null value

( http://www.scala-lang.org/files/archive/spec/2.11/03-types.html#volatile-types )

但这究竟有什么问题呢？是否有使用 @uncheckedStable 的示例？ (见 http://www.scala-lang.org/files/archive/spec/2.11/11-annotations.html#scala-compiler-annotations )产生不安全代码？

Answer 1

object Main extends App {      
  trait A { type T = Int }
  trait B { type T <: String }
  def f(b: B)(t: b.T) = t.length

  @annotation.unchecked.uncheckedStable val x: A with B = null
  val y: x.T = 0 // legal because x is A

  f(x)(y)
}

Now running...
[info] Running Main 
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

(基于retronym 对 Cannot override a type with non-volatile upper bound 的回答。)