Missing unchecked warning for isInstanceOf checks

[allanrenucci]

This code snippet emits a warning with scalac but not with Dotty

class Foo {
  def foo(x: Any): Boolean =
    x.isInstanceOf[List[String]]
}

Foo.scala:3: warning: non-variable type argument String in type List[String] (the underlying of List[String]) is unchecked since it is eliminated by erasure
    x.isInstanceOf[List[String]]
                  ^
one warning found

[odersky]

I am not sure what we want to spec here, yet.

[odersky]

I now think we should do the same as scalac.

[allanrenucci]

For (x: X).isInstanceOf[P], scalac emits an unchecked warning when:

We designate another type, XR, which is essentially the intersection of X and |P|, where |P| is the erasure of P. If XR <: P, then no warning is emitted.

ref: https://github.com/scala/scala/blob/94cee79eb63b4449d21dc74cb9175d6412cc8a55/src/compiler/scala/tools/nsc/typechecker/Checkable.scala#L12-L64

[odersky]

It's actually a wider problem: Consider:

class C[T] {
  val x: Any = ???
  if (x.isInstanceOf[List[String]]) // error: unchecked
    if (x.isInstanceOf[T])          // error: unchecked
      x match {
        case x: List[String] =>     // error: unchecked
        case x: T =>                // error: unchecked
      }
}

scalac will issue an unchecked warning at each line. Dotty remains silent.