gistfile1.scala

trait MyTrait{
  def say: String
}

trait ConcreteTrait extends MyTrait{
  def say = "Hello, world"
}

trait TestTrait extends MyTrait{
  def say = "Testing"
}

class Foo{
  this: MyTrait =>
  println(say)
}

val x = new Foo // compilation error
val y = new Foo with ConcreteTrait // works
val test = new Foo with TestTrait // for testing