Why you should always declare case classes final

final-case-class.scala

case class X()

sealed trait Unrelated { def x: Int }

object App extends App {
 
  def getX(xs: List[X]): List[Int] = {
    xs.collect {
      case u: Unrelated => u.x
    }
  }

}

// getX's `collect` is non-sensical, it can never match,
//   but Scalac lets it through: https://scastie.scala-lang.org/68FQ2TIDSDyQJuigOHN4Ow
// However, if we declare `X` final,
//   Scalac will error out on `case u: Unrelated` and prevent nonsense: https://scastie.scala-lang.org/owueF0FnQJOrCP43ugxQfQ