package tree

abstract class Tree {

  type FinalBranch <: Branch

  trait Branch {
    this: FinalBranch =>
    def myself: FinalBranch = this
  }

}

object Matcher {
  def unapply[T <: Tree](branch: T#Branch) = Some(branch.myself)
}

object Helper {
  def itself[T <: Tree](branch: T#Branch) = branch.myself
  def optionItself[T <: Tree](branch: T#Branch) = Some(branch.myself)
}

object Oak extends Tree {
  type FinalBranch = Branch
  class Branch extends super.Branch {
    this: FinalBranch =>
  }
}

object Caller extends App {
  val oakBranch = new Oak.Branch
  println(oakBranch.getClass()) // <-- class tree.Oak$Branch
  val naked = Helper.itself(oakBranch)
  println(naked.getClass()) // <-- class tree.Oak$Branch
  val unwrapped = Helper.optionItself(oakBranch).get
  println(unwrapped.getClass()) // <-- class tree.Oak$Branch
  val matched = oakBranch match {
    case Matcher(branch) => branch // <-- type mismatch; found: T#Branch; required: Oak.Branch
  }
}

$ scalac tree/Tree.scala 
tree/Tree.scala:38: error: type mismatch;
 found   : T#Branch
 required: tree.Oak.Branch
    case Matcher(branch) => branch // <-- type mismatch; found: T#Branch; required: Oak.Branch
                ^
one error found