Learning about generic types that need to access their subtypes - in Scala

FBounded.scala

trait Color {
  def name: String
}

abstract class PrimaryColor(val name: String) extends Color
case object Red extends PrimaryColor("Red")
case object Blue extends PrimaryColor("Blue")
case object Yellow extends PrimaryColor("Yellow")

trait HasColors {
  def colors: collection.immutable.Set[Color]
}

/** http://twitter.github.io/scala_school/advanced-types.html#fbounded */
abstract class Paint[P <: Paint[P]] extends HasColors {
  def name: String
  def price: Double
  def id: Int
  def withColor(color: Color): Paint[P] = {
    _colors += color
    this
  }
  private val _colors = collection.mutable.Set.empty[Color]
  def colors = _colors.toSet
  def lineItem: String = s"[${id}] ${name}${colors.mkString("(",",",")")}: $$${price}"
}

case class OilPaints(id: Int, price: Double) extends Paint[OilPaints] {
  val name = "Oils"
}
case class WaterColors(id: Int, price: Double) extends Paint[WaterColors] {
  val name = "WaterColors" 
}

object Main extends App {
  println("******************")
  val oil = OilPaints(100, 1.0).withColor(Red).withColor(Blue)
  val wc = WaterColors(200, 2.0).withColor(Yellow)
  println(oil.lineItem)
  println(wc.lineItem)

  //val broken: WaterColor = OilPaint(3.0).withColor(Red)

  println("******************")
  val p: Paint[_] = WaterColors(300, 4.0)
  println(p.lineItem)

  println("******************")
  println("Printing list:")
  val paints = OilPaints(400, 1.0) :: 
    WaterColors(500, 2.0).withColor(Yellow) ::
    OilPaints(600, 3.0).withColor(Red) :: 
    OilPaints(700, 4.0).withColor(Yellow).withColor(Blue) :: Nil

    paints.foreach(p => println(p.lineItem))
}

// OUTPUT:
// ******************
// [100] Oils(Red,Blue): $1.0
// [200] WaterColors(Yellow): $2.0
// ******************
// [300] WaterColors(): $4.0
// ******************
// Printing list:
// [400] Oils(): $1.0
// [500] WaterColors(Yellow): $2.0
// [600] Oils(Red): $3.0
// [700] Oils(Blue,Yellow): $4.0

scala> val b = OilPaints(100, 1) :: WaterColors(200, 2.0) :: Nil
b: List[org.example.Paint[_ >: org.example.WaterColors with org.example.OilPaints <: org.example.Paint[_ >: org.example.WaterColors with org.example.OilPaints <: Product with Serializable with org.example.HasColors] with Product with Serializable] with Product with Serializable] = List(OilPaints(100,1.0), WaterColors(200,2.0))

Gah!