http://codr.cc/356776/rb を見て何となく

gistfile1.scala

scala> sealed trait Color
defined trait Color

scala> case object Black extends Color
defined module Black

scala> case object Pink extends Color
defined module Pink

scala> case class Pants(color: Color)
defined class Pants

scala> trait HasLegs
defined trait HasLegs

scala> class Oneechan(val pants: Pants) extends HasLegs
defined class Oneechan

scala> val homuhomu = new Oneechan(Pants(Pink))
homuhomu: Oneechan = Oneechan@1baa8d8

scala> homuhomu.pants
res0: Pants = Pants(Pink)

scala> homuhomu.pants = Pants(Black)
<console>:15: error: reassignment to val
       homuhomu.pants = Pants(Black)
                      ^

scala> trait Tights { this: HasLegs =>
     |   def tightsColor: Color
     | }
defined trait Tights

scala> val homuhomu = 
     |   new Oneechan(Pants(Black)) with Tights {
     |     def tightsColor = Black
     |   }
homuhomu: Oneechan with Tights{def tightsColor: Black.type} = $anon$1@12a09d5

scala> homuhomu.tightsColor
res1: Black.type = Black

scala> class Incubator
defined class Incubator

scala> val qb =
     |   new Incubator with Tights {
     |     def tightsColor = Black
     |   }
<console>:13: error: illegal inheritance;
 self-type Incubator with Tights does not conform to Tights's selftype Tights with HasLegs
         new Incubator with Tights {
                            ^