Imported From: https://issues.scala-lang.org/browse/SI-9419?orig=1
Reporter: Stephen Compall (s11001001)
Affected Versions: 2.10.5, 2.11.7
See #9410

Still broken in Scala 3.

context: https://typelevel.org/blog/2015/07/30/values-never-change-types.html

Dale and I noticed that it isn't crucial to have an actual byname; you can change the reproducer to use () => Magic[T] instead of => Magic[T] and it still breaks in the same way.

(Regardless, we should test any proposed fix on both variants.)

I'm partial to this minimization (in Scala 3 syntax):

trait Magic[S]:
  def init: S
  def step(s: S): String

object IntMagic extends Magic[Int]:
  def init = 0
  def step(s: Int): String = (s - 1).toString

object StrMagic extends Magic[String]:
  def init = "hi"
  def step(s: String): String = s.reverse

object Main:
  def onestep[T](m: () => Magic[T]): String = m().step(m().init)
  val iter: Iterator[Magic[?]] = Iterator(IntMagic, StrMagic)
  def main(args: Array[String]): Unit =
    onestep(() => iter.next())

it's not fundamentally different. Just a bit easier to grasp, perhaps.

Magic is invariant and has methods with S in both covariant and contravariant positions, as is common when we want a runtime demonstration of compile-time unsoundness.

onestep(() => iter.next()) should not compile, as there no correct instantiation for T. (Recall that Iterator[Magic[?]] means Iterator[Magic[T] forSome { type T }], not Iterator[Magic[T]] forSome { type T }.)

scala/scala3#16732 is merged, so, labeling this one as "fixed in Scala 3" now

UPDATE: see also discussion at scala/scala3#16789

errant auto-closing. not fixed in Scala 2.