No warning on uncheckable isInstanceOf call

Minimized code

import scala.Option
object Test extends App {
  trait A[+X]
  class B[+X](val x: X) extends A[X]
  object B {
    def unapply[X](b: B[X]): Option[X] = Some(b.x)
  }

  class C[+X](x: Any) extends B[Any](x) with A[X]
  def f(a: A[Int]): Int =
    if a.isInstanceOf[B[Int]] then
      a.asInstanceOf[B[Int]].x
    else
      0
  f(new C[Int]("foo"))
}

Output

Compiles without warning, explodes at runtime.

Expectation

A warning about the .isInstanceOf test should be emitted.

Adapted from https://github.com/lampepfl/dotty/blob/master/tests/neg/i3989c.scala#L1.

/cc @liufengyun it seems that additional tweaking of how GADTs are used for checking .isInstanceOf is necessary.

[dwijnand]

@abgruszecki discovered that Kotlin also requires “invariant refinements”, in the following gist covariant type parameter T (“out” means covariant) is refined from Any to Int, and that is a compilation error:

https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS42LjIxIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncyI6IiIsIm5vbmVNYXJrZXJzIjp0cnVlLCJ0aGVtZSI6ImlkZWEiLCJjb2RlIjoiLyoqXG4gKiBZb3UgY2FuIGVkaXQsIHJ1biwgYW5kIHNoYXJlIHRoaXMgY29kZS5cbiAqIHBsYXkua290bGlubGFuZy5vcmdcbiAqL1xuaW50ZXJmYWNlIEJhc2UgPG91dCBUPjtcbmludGVyZmFjZSBCYXNlQW55IDogQmFzZTxBbnk+O1xuaW50ZXJmYWNlIEJhc2VJbnQgOiBCYXNlPEludD47XG5vcGVuIGNsYXNzIERlcml2ZWQxKCkgOiBCYXNlIDxBbnk+O1xuY2xhc3MgRGVyaXZlZDIoKSA6IERlcml2ZWQxKCkgLCBCYXNlSW50OyJ9

[odersky]

Perhaps it’s just not doable with baseType and I need to look to do something like paramInfoAsSeenFrom? Any tips or advice welcome.

I think we are all still scratching our heads way too much to be able to give advice on specifics like that. 🤯