Another form of unboxed tagged types.

Tagged.scala

import language.higherKinds

object Tagged {
  type @@[A, B] = { type T = A }

  type Id[A] = A

  implicit class Taggable[A](a: A) {
    def tagged[B]: A @@ B = a.asInstanceOf[A @@ B]
  }

  implicit class Untaggable[A, B](a: A @@ B) {
    def &(f: A @@ B => A @@ B): A @@ B = f(a)
    def untagged: A = a.asInstanceOf[A]
  }

  trait ~>[F[_], G[_]] {
    def apply[A](fa: F[A]): G[A]
  }

  trait IsoFunctor[T, F[_], G[_]] { self =>
    def to: F ~> G
    def from: G ~> F
    def %~[A](f: G[A] => G[A]): F[A] => F[A] = a => from(f(to(a)))
    def flipped = new IsoFunctor[T, G, F] {
      val to = self.from
      val from = self.to
    }
  }

  object TaggedIso {
    def apply[T] = new IsoFunctor[T, ({type l[A]=A @@ T})#l, Id] {
      val to = new (({type l[A]=A @@ T})#l ~> Id) {
        def apply[A](fa: A @@ T): Id[A] = fa.untagged
      }
      val from = new (Id ~> ({type l[A]=A @@ T})#l) {
        def apply[A](fa: Id[A]): A @@ T = fa.tagged
      }
    }
  }
}

object Example {
  import Tagged._

  trait Age
  val brianAge = 10.tagged[Age] & TaggedIso[Age] %~ (_ + 13)
}