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.