Typer: Incorrent assumtions when typing pattern matches

[DarkDimius]

Typer assumes that pattern matching in only doing dowcasts and rejects this valid code. This code is accepted by scalac:

trait A
trait B

abstract sealed class Parent
class Foo extends Parent with A
class Bar extends Parent with B

(null: Parent) match {
  case (_: A) | (_: B) => // dotty complains here that neither A or B are subtypes of Parent
}

[DarkDimius]

Decided that:

1. we do not want to substantially change the current behaviour
2. we would like to have a simplistic mode for -languge:Scala2 that typechecks the subpattern with either lub or some parent type as expected type.

[odersky]

I just looked at it. Typer accepts this code. The error gets raised by IsInstanceOfEvaluator. Reassigning to Felix.