trait Module {
  sealed trait Tree

  case class LetL() extends Tree
}

class Patmat[T <: Module](val module: T) {
  def patmat(tree: module.Tree) = tree match {
    case module.LetL() => ???
  }
}

patmat-acc.scala:8: warning: match may not be exhaustive.
It would fail on the following input: LetL()
  def patmat(tree: module.Tree) = tree match {
                                  ^
one warning found

class Patmat[T <: Module](val module: T)

class Patmat(val module: Module)