trait Tree
sealed abstract class Prop
trait Simple extends Prop
case class Atom(tree: Tree) extends Prop with Simple
case class Not(prop: Prop) extends Prop with Simple

object Test {
  def main(args: Array[String]): Unit = {
    def simplify(prop: Prop): Prop = prop match {
      case Not(Atom(tree)) => ???
      case Not(Not(prop)) => ???
      case _ => ???
    }
  }
}

14:20 ~/Projects/2117/sandbox (HEAD)$ s
Test.scala:11: warning: unreachable code
      case Not(Not(prop)) => ???
                             ^
one warning found