No warning on typecase pattern involving a case class

[abgruszecki]

Minimized code

   trait Box[+T]
   case class Foo[+S](s: S) extends Box[S]

   def unwrap2[A](b: Box[A]): A =
     b match
     case _: Foo[Int] => 0 // compiles, not ok

Output

Compiles with no warning, which is not OK.

Expectation

At least a warning about a pattern that cannot be checked at runtime should be emitted. The code shouldn't typecheck as well, but that's a secondary problem.

[liufengyun]

Seems related to #4298

[adpi2]

I came across the same error and I found that a warning is issued if the type is invariant but not if it's covariant or contravariant:

// Invariant case, OK
sealed trait Bar[A]

def test[A](bar: Bar[A]) =
  bar match {
    case _: Bar[Boolean] => ??? // unchecked warning, OK
    case _ => ???
  }

// Covariant case, not OK
sealed trait Bar[+A]

def test[A](bar: Bar[A]) =
  bar match {
    case _: Bar[Boolean] => ??? //  no warning, not OK
    case _ => ???
  }

// Contravariant case, not OK
sealed trait Bar[-A]

def test[A](bar: Bar[A]) =
  bar match {
    case _: Bar[Boolean] => ??? // no warning, not OK
    case _ => ???
  }