Cast to Null$ in pattern match involving existential type

[lrytz]

In both 2.12.17 and 2.13.10

object Test {
  trait MFSS[Self <: MFSS[Self]]
  trait CS extends MFSS[CS]

  trait MFI {
    type ST
  }

  case class MFSD[S <: MFSS[S]](mFI: MFI { type ST = S })

  case object IOS extends MFI {
    type ST = CS
  }

  type SD = MFSD[S] forSome {
    type S <: MFSS[S]
  }

  def bad(sd: SD) = sd.mFI match {
    case ios : IOS.type => println(ios)
  }

  def good1(sd: SD) = (sd.mFI: MFI) match {
    case ios : IOS.type => println(ios)
  }

  def good2(sd: SD) = sd.mFI match {
    case ios @ IOS => println(ios)
  }


  def main(args: Array[String]): Unit = {
    val x = MFSD(IOS)
    good1(x)
    good2(x)
    bad(x)
  }
}

➜ sandbox sc Test.scala && s Test
warning: 1 feature warning; re-run with -feature for details
1 warning
IOS
IOS
java.lang.ClassCastException: class Test$IOS$ cannot be cast to class scala.runtime.Null$ (Test$IOS$ and scala.runtime.Null$ are in unnamed module of loader java.net.URLClassLoader @437da279)
	at Test$.bad(Test.scala:20)
	at Test$.main(Test.scala:36)

[lrytz]

Added a test case for the underlying bug here: https://github.com/scala/scala/compare/2.13.x...lrytz:scala:t12702-pending?expand=1

Not sure how to fix that for now...