https://stackoverflow.com/questions/52660723/aux-pattern-with-inherited-types-fails-inference

App.scala

object App {
  trait Subtype[A, B] {
    type C <: B
    def apply(ev: A <:< B): (C =:= A, A =:= C)
  }

  object Subtype {
    type Aux[A, B, C0 <: B] = Subtype[A, B] { type C = C0 }
    def instance[A, B, C0 <: B](f: A <:< B => (C0 =:= A, A =:= C0)): Aux[A, B, C0] = new Subtype[A, B] {
      override type C = C0
      override def apply(ev: A <:< B): (C0 =:= A, A =:= C0) = f(ev)
    }

    def materialize[A, B](implicit subtype: A Subtype B): Aux[A, B, subtype.C] = subtype
    def materialize1[A, B](ev: A <:< B)(implicit subtype: A Subtype B): Aux[A, B, subtype.C] = subtype

    implicit def mkSubtype[A, B >: A]: Aux[A, B, A] = instance(_ => (implicitly, implicitly))
  }

  val x = Subtype.materialize[Int, AnyVal]
  implicitly[x.C <:< AnyVal]
  x(implicitly)._1: x.C =:= Int
  x(implicitly)._2: Int =:= x.C
  def test[A, B, C <: B] = ???
  test[Int, AnyVal, x.C]

  val y = Subtype.materialize1(implicitly[Int <:< AnyVal])
  implicitly[y.C <:< AnyVal]
  y(implicitly)._1: y.C =:= Int
  y(implicitly)._2: Int =:= y.C
  test[Int, AnyVal, y.C]

}