object App { def main(args: Array[String]): Unit = { trait AgeT { type T def subst[F[_]](fa: F[Int]): F[T] } type Age = Age.T final val Age: AgeT = new AgeT { type T = Int def subst[F[_]](fa: F[Int]): F[T] = fa } sealed abstract class K[A] final case object KAge extends K[Age] final case object KInt extends K[Int] val kint: K[Age] = Age.subst[K](KInt) def get(k: K[Age]): String = k match { case KAge => "Age" } get(kint) } }