Imported From: https://issues.scala-lang.org/browse/SI-8737?orig=1
Reporter: @odersky
Affected Versions: 2.11.1
Attachments:

• variances.scala (created on Jul 20, 2014 12:57:22 PM UTC, 847 bytes)

@retronym said:
From a re-report, #9549:

class C[+A] {
 
  private[this] var y: A = _
  def getY: A = y
 
  class Inner(x: A) {
    y = x
  }
}
 
object Test {
 
  def main(args: Array[String]) = {
    val x = new C[String]
    val y: C[Any] = x
    val i = new y.Inner(1)
    val s: String = x.getY
    println(s)
  }
}

Chung-Kil Hur (Gil) said (edited on Sep 26, 2016 4:16:14 AM UTC):
Dear developers,

The following code also seems to show a related unsoundness.

sealed abstract class MyADT[+A]
case object MyNone extends MyADT[Nothing]
case class MyFun[A](fun: A=>A) extends MyADT[A]

def data : MyADT[Any] = MyFun((x:Int)=>x+1)

val foo : Any =
  data match {
    case MyFun(f) => f("a")
    case _ => 0
  }

In this code, I simply try to encode the following polymorphic ADT in such a way that MyNone does not require unnecessary parentheses ().

MyADT[A] = MyNone | MyFun of A->A

However, as the code shows, Scala unsoundly accepts the wrong code and produce a type error at run time.
So, could you let me know whether there is a sound way to encode MyADT without requiring unnecessary parentheses in Scala?

Thanks,
Gil