In Scala, why unchecked warning is not issued for type arguments?

Question

I understand generally what type erasure is and why we would encounter unchecked warnings. However, I don't understand why only one unchecked warning is issued in the following case:

class A[K] {
  def receive: PartialFunction[Any, Unit] = {
    case ds: List[Double] =>  // unchecked warning
      println("* List[Double]")
    case kx: Vector[K] =>      // no unchecked warning
      println("* Vector[K]")
  }
}

object TestApp extends App {
  val a = new A[Int]
  a.receive(List("bar"))
  a.receive(Vector("foo"))
}

Both receive calls unfortunately match case clauses. Compiler did issue an warning for the first clause:

Warning: non-variable type argument Double in type pattern List[Double] is unchecked since it is eliminated by erasure.

I know TypeTag[T] can be used to achieve better type safety. But my concern here is why no unchecked warning is issued for the second case clause. As far as I think, type argument K is also erased and according to Java Generics FAQ

"unchecked" warnings are also reported when the compiler finds a cast whose target type is either a parameterized type or a type parameter

So I wonder why isn't there an unchecked warning?

Answer 1

It's probably a bug. SI-9188 is open for that.