A.scala

sealed abstract class Y

class Y1 extends Y
class Y2 extends Y

sealed abstract class X[B](val value: Int)

case object X1 extends X[Y1](1)
case object X2 extends X[Y2](2)

object A {
  val values: Seq[X[Y]] = ???

  def get(value: Int): X[Y] = {
    values.find(_.value == value) match {
      case Some(v) => v
      case _ => sys.error("not found")
    }
  }
}

log.txt

[warn] 16 |      case Some(v) => v
[warn]    |           ^^^^^^^
[warn]    |           Unreachable case
[warn] one warning found