Type erased after pattern matching

[Atry]

Welcome to Scala 2.12.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131).
Type in expressions for evaluation. Or try :help.

scala> Some(1)  match {
     | case s: Option[_] =>
     | s
     | } 
res0: Some[Int] = Some(1)

scala> Some(1)  match {
     | case s: Some[_] =>
     | s
     | } 
res1: Some[Any] = Some(1)

[hongxuchen]

Perhaps because Some[A] is co-variant and s: Some[_] will be matched conservatively as to existential type? If otherwise it is invariant, type will be kept as expected.

sealed trait T[+A]

object T {
  def apply[A](x: A): T[A] = if (x == null) S2 else S1(x)
}

case class S1[A](v: A) extends T[A] // invariance

case object S2 extends T[Nothing]

S1(1) match {
  case s1: S1[_] => s1
}
// S1[Int] = S1(1)

[SethTisue]

Scala 3.3.1 does a better job here:

scala> Some(1) match { case s: Some[_] => s }
val res1: Some[Int] = Some(1)