gistfile1.scala

sealed trait Base[T] {
   val s: Option[String] 
   def copy(s: Option[String]) : T
}

case class CaseClass(override val s: Option[String] = None) extends Base[CaseClass] {
    override def copy(s: Option[String]) = CaseClass(s)    
}

def method[T <: Base[T]](base : Base[T]): T = {
   base copy (s = Some("strng"))
}