enum.scala // https://stackoverflow.com/questions/25838411/cant-prove-that-singleton-types-are-singleton-types-while-generating-type-class

enum.scala

import shapeless._

trait AllSingletons[A, C <: Coproduct] {
  def values: List[A]
}

object AllSingletons {
  implicit def cnilSingletons[A]: AllSingletons[A, CNil] =
    new AllSingletons[A, CNil] {
      def values = Nil
    }

  implicit def coproductSingletons[A, H <: A, T <: Coproduct](implicit
    tsc: AllSingletons[A, T],
    witness: Witness.Aux[H]
  ): AllSingletons[A, H :+: T] =
    new AllSingletons[A, H :+: T] {
      def values = witness.value :: tsc.values
    }
}

sealed trait Foo
case object Bar extends Foo
case object Baz extends Foo

trait EnumerableAdt[A] {
  def values: Set[A]
}

object EnumerableAdt {
  implicit def fromAllSingletons[A, C <: Coproduct](implicit
    gen: Generic.Aux[A, C],
    singletons: AllSingletons[A, C]
  ): EnumerableAdt[A] = {
    val _ = gen
    new EnumerableAdt[A] {
      def values = singletons.values.toSet
    }
  }
}

object Main extends App {
  val x = implicitly[AllSingletons[Foo, Bar.type :+: Baz.type :+: CNil]]
  println(x.values)

  val y = implicitly[EnumerableAdt[Foo]]
  println(y.values)
}