Demonstrates a scenario where Scala fails to refine a type

roles.scala

sealed trait Role[N <: Entity] 

case object VIP extends Role[Customer]
case object Shopper extends Role[Customer]

case object Manager extends Role[Staff]
case object Employee extends Role[Staff]

sealed trait Entity

case class Customer(name: String) extends Entity
case class Staff(id: Int) extends Entity

sealed trait RolePair {
  type N <: Entity
  val role: Role[N]
  val entity: N
}

object RolePair {
  def apply[M <: Entity](r: Role[M], n: M): RolePair = new RolePair {
    type N = M
    val role: Role[N] = r
    val entity: N = n
  }

  def unapply(rp: RolePair): Some[(Role[rp.N], rp.N)] = Some((rp.role, rp.entity))
  // Scala 2 version (broken in 2.11, 2.12)
  // def unapply(rp: RolePair): Some[(Role[N], N)] forSome { type N <: Entity } = Some((rp.role, rp.entity))
}

object RolePairTest {
  def main(args: Array[String]): Unit = {
    val roles: List[RolePair] = List(
      RolePair(Manager, Staff(1)),
      RolePair(Shopper, Customer("Max")),
      RolePair(Employee, Staff(2)),
      RolePair(Employee, Staff(3)),
      RolePair(VIP, Customer("Lucy"))
    )

   // The following hould be a List[Customer], but it is only a List[RolePair#N] under dotty (List[Entity] under 2.11, 2.12)
   // This worked in 2.10 but is broken in 2.11 and dotty
   val vips = 
     roles collect {
       case RolePair(VIP, customer) => customer
     }

   vips foreach { vip => println(vip.name) }
    
   val vipsAndManagers = // Should be a List[Customer | Manager], but it is only a List[RolePair#N]
     roles collect {
       case RolePair(VIP, customer) => customer
       case RolePair(Manager, manager) => manager
     }

  }
}

Note that I can't even call .name within the body of the case statement on line 47.