1.scala

trait SourceInfo {
  type NoInfoType <: this.type
  val NoInfo: NoInfoType
  def showMessage(msg: String): String
}

object NoPositionInfo extends PositionInfo(NoPosition) {
  override def showMessage(msg: String): String = s"${pos.toString}: $msg"
}

class PositionInfo(val pos: Position) {
  type NoInfoType = NoPositionInfo.type

  val NoInfo = NoPositionInfo

  def showMessage(msg: String): String = s"${pos.toString}: $msg\n${pos.longString}"
}

2.scala

trait SourceInfo {
  type NoInfoType <: this.type
  val NoInfo: NoInfoType
  def showMessage(msg: String): String
}

object NoPositionInfo extends PositionInfo(NoPosition) with SourceInfo {
  override def showMessage(msg: String): String = s"${pos.toString}: $msg"
}

class PositionInfo(val pos: Position) {
  self: SourceInfo =>

  type NoInfoType = NoPositionInfo.type

  val NoInfo = NoPositionInfo

  def showMessage(msg: String): String = s"${pos.toString}: $msg\n${pos.longString}"
}