Pattern matching on a local class is unsound, should emit an unchecked warning

Adapted from http://wouter.coekaerts.be/2018/java-type-system-broken which demonstrates the same thing in Java:

object Test {
  var prev: Any = _
  def test[T](x: T): T = {
    class A(val elem: T)
    if (prev == null) {
      val a = new A(x)
      prev = a
      x
    } else {
      prev match {
        case prev: A => // This should warn, but doesn't
          prev.elem
        case _ =>
          x
      }
    }
  }

  def main(args: Array[String]): Unit = {
    test(1)
    val x: String = test("") // ClassCastException: java.lang.Integer cannot be cast to java.lang.String
  }
}

There’s no way to distinguish instances of a local class coming from different method calls, so the type test is never safe and should always require an @unchecked annotation.

[dwijnand]

It’s better now with https://github.com/lampepfl/dotty/pull/16086

[SethTisue]

(It would be nice if the error message gave some clue as to the specific nature of the problem. I wouldn’t have guessed this — neither @WojciechMazur nor I guessed it at today at #16259.)