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I have two classes PixelObject , ImageRefObject and some more, but here are just these two classes to simplify things. They all are subclasses of a trait Object that contains an uid. I need universal method which will copy a case class instance with a given new uid . The reason I need it because my task is to create a class ObjectRepository which will save instance of any subclass of Object and return it with new uid . My attempt: trait Object { val uid: Option[String] } trait UidBuilder[A <: Object] { def withUid(uid: String): A = { this match { case x: PixelObject => x.copy(uid = Some(uid)) case x: ImageRefObject => x.copy(uid = Some(uid)) } } } case class PixelObject(uid: Option[String], targetUrl: String) extends Object with UidBuilder[PixelObject] case class ImageRefObject(uid: Option[String], targetUrl: String, imageUrl: String) extends Object with UidBuilder[ImageRefObject] val pix = PixelObject(Some("oldUid"), "http://example.com") val newPix = pix.withUid("newUid") println(newPix.toString) but I am getting the following error: ➜ ~ scala /tmp/1.scala /tmp/1.scala:9: error: type mismatch; found : this.PixelObject required: A case x: PixelObject => x.copy(uid = Some(uid)) ^ /tmp/1.scala:10: error: type mismatch; found : this.ImageRefObject required: A case x: ImageRefObject => x.copy(uid = Some(uid)) ^ two errors found

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