Trying to apply shapeless LiftAll instances to an HList

LiftAllZipping.scala

import shapeless._
import ops.hlist.LiftAll
import shapeless._
import syntax.singleton._
import shapeless.ops.hlist.ZipWithKeys

case class SingletonOf[T, U](value: U)

object SingletonOf {
  implicit def mkSingletonOf[T <: AnyRef](implicit t: T): SingletonOf[T, t.type] = SingletonOf(t)
}

trait Showable[T] {
  def show(t: T): String
}

object Showable {
  implicit def stringShowable = new Showable[String] {
    def show(t: String) = s"String: $t"
  }
}

def test[L <: HList, U <: { type Out <: HList }](l: L)(
  implicit
  zippable: ZipWithKeys[L, U#Out],
  showable: LiftAll[Showable, L],
  allShowable: SingletonOf[LiftAll[Showable, L], U]
) =
  showable.instances.asInstanceOf[U#Out].zipWithKeys(l)
test("foo" :: HNil)