scala - rely on methods of case class in trait

Question

Is there a way to rely on methods defined in case class in a trait? E.g., copy: the following doesn't work. I'm not sure why, though.

trait K[T <: K[T]] {
  val x: String
  val y: String
  def m: T = copy(x = "hello")
  def copy(x: String = this.x, y: String = this.y): T
}

case class L(val x: String, val y: String) extends K[L]

Gives:

error: class L needs to be abstract, since method copy in trait K of type 
(x: String,y: String)L is not defined
           case class L(val x: String, val y: String) extends K[L]
                      ^

Answer 1

A solution is to declare that your trait must be applied to a class with a copy method:

trait K[T <: K[T]] {this: {def copy(x: String, y: String): T} =>
  val x: String
  val y: String
  def m: T = copy(x = "hello", y)
}

(unfortunately you can not use implicit parameter in the copy method, as implicit declaration is not allowed in the type declaration)

Then your declaration is ok:

case class L(val x: String, val y: String) extends K[L]

(tested in REPL scala 2.8.1)

The reason why your attempt does not work is explained in the solution proposed by other users: your copy declaration block the generation of the "case copy" method.