Implicit val can't be annotated with any type so that the code compiles https://stackoverflow.com/questions/55548968/how-to-use-functors-with-scala-cats/55549231#comment97803502_55549231

App.scala

object App {

  trait MyTrait
  object MyTrait {
    def materialize[A <: MyTrait](implicit m: A): A = m
  }

  implicit val m/*: MyTrait*/ /*: (MyTrait {
    /*implicit*/ val x: Int
    })*/ = new MyTrait {
    implicit val x: Int = 10
  }

  val m1 = MyTrait.materialize
  import m1._
  implicitly[Int]

}

import shapeless.{::, HList, HNil, Poly2}

object App {

  def swap(p: Poly2)/*: Poly2*/ /*: (Poly2 {
    /*implicit*/ def cse[L <: HList, A, B](implicit
                                       ev: L <:< (A :: B :: HNil),
                                       pCase: p.Case[B, A]
                                      ): ProductCase.Aux[L, pCase.Result]
  })*/ = new Poly2 {
    implicit def cse[L <: HList, A, B](implicit
                                       ev: L <:< (A :: B :: HNil),
                                       pCase: p.Case[B, A]
                                      ): ProductCase.Aux[L, pCase.Result] = ProductCase(l => {
      val a :: b :: HNil = ev(l)
      pCase(b, a)
    })
  }

  object myPoly extends Poly2 {
    implicit val intCase: Case.Aux[Int, Boolean, Long] = at((i, _) => i.toLong)
    implicit val strCase: Case.Aux[String, Char, Symbol] = at((s, c) => Symbol(s + c))
  }

  val myPoly1 = swap(myPoly)
  import myPoly1._
  myPoly1(true, 10)
  myPoly1('c', "ab")
}