Having a bit of a problem deriving implicit instances for product types.

gistfile1.scala

import shapeless._

// typeclass for total number of fields (just an example)
class Width[A](val toInt: Int)

object Width {

  // Get an instance
  implicit def apply[A](implicit A: Width[A]) = A

  // now derive for HList
  implicit val fnil: Width[HNil] = 
    new Width[HNil](0)

  implicit def fcons[H, T <: HList ](implicit H: Width[H], T: Width[T]): Width[H :: T] = 
    new Width[H :: T](H.toInt + T.toInt)

  // and for products generally
  implicit def fprod[P <: Product, L <: HList](implicit PL: Generic.Aux[P, L], L: Width[L]): Width[P] = 
    new Width[P](L.toInt)

}

// some instances
implicit val fooInt = new Width[Int](1)
implicit val fooStr = new Width[String](1)

Width[Int].toInt // 1
Width[Int :: String :: HNil].toInt // 2
Width[(Int, (String, String))].toInt // 3
Width[(Int, ((String, Int), String))].toInt // 4

// other products
case class Once(name: String)
case class Twice(f1: Once, f2: Once) // works as (Once, Int) for example

Width[Once].toInt // 1
Width[Twice].toInt // 2


Width[(Once, Int)].toInt // 2

Width[(Twice, Int)].toInt
// <console>:25: error: diverging implicit expansion for type Width[(Twice, Int)]
// starting with method fprod in object Width
//              Width[(Twice, Int)].toInt