compile error scalaz with Scala 2.13.2 ( "covariant type A occurs in invariant position")

[xuwei-k]

reproduction steps

git clone git://github.com/scalaz/scalaz.git
cd scalaz
git checkout scala-2.13.2
sbt ++2.13.2 coreJVM/compile

problem

• https://travis-ci.com/github/scalaz/scalaz/jobs/322570510#L608
• https://github.com/scalaz/scalaz/blob/9a5df84b6bd9ac762d2aa52075c8d136cb05895b/core/src/main/scala/scalaz/IList.scala#L507-L513

[error] /home/travis/build/scalaz/scalaz/core/src/main/scala/scalaz/IList.scala:510:29: covariant type A occurs in invariant position in type [+A]scalaz.INil[A] {
[error]   def <init>(): scalaz.INil._INil[A]
[error] } of class _INil
[error]   private[this] final class _INil[+A] extends INil[A]
[error]                             ^

expectation

not compile error.

[SethTisue]

@joroKr21 might this have to do with scala/scala#8545 ?

[xuwei-k]

more simpler example

package example

sealed trait IList[A]

sealed abstract case class INil[A] () extends IList[A]
object INil {
  private[this] final class _INil[+A] extends INil[A]
}

[smarter]

This is unsound in general so scalac is right to reject this code:

sealed trait IList[A]

sealed abstract case class INil[A]() extends IList[A] { var elem: A = _ }
object INil {
  private[this] final class _INil[+A] extends INil[A]

  def main(args: Array[String]): Unit = {
    val n = new _INil[Int]
    (n: _INil[Any]).elem = "foo"
    val i: Int = n.elem // ClassCastException
  }
}

[smarter]

(you should be able to use @uncheckedVariance to bypass this)

[xuwei-k]

Thank you for quick response!

[joroKr21]

What @smarter said 😄