Forcing invariance with compiler-generated traits and classes.

PhantomDiamond.scala

trait Immutable

class A

class B extends A

sealed trait AImm extends A with Immutable

sealed trait BImm extends B with AImm with Immutable

final class APrim extends AImm

final class BPrim extends BImm

object Test {
    var a: A = new A()
    var b: A = new B()
    var c: A = new APrim()
    var d: A = new BPrim()

    //Works
    var dd: B = new BPrim()
    var ddd: AImm = new BPrim()
    var dddd: BImm = new BPrim()

    //Does not work
    //var ddddd: APrim = new BPrim()
}

PhantomList.scala

trait Immutable

abstract class Maybe[+A]

case class Ingenting() extends Maybe[Nothing]
case class Just[+A](value: A) extends Maybe[A]


sealed trait OptionImm[+A] extends Maybe[A]

final class IngentingImm extends Ingenting with OptionImm[Nothing]
final class JustImm[+A](value: A) extends Just[A](value) with OptionImm[A]

zNotes.md

Covariance and contravariance rules in Java

The overriding method is covariant in the return type and invariant in the argument types. That means that the return type of the overriding method can be a subclass of the return type of the overridden method, but the argument types must match exactly.