SI2712.scala

import cats.Id
import cats.data.{Const, Prod}

/**
  *
  *
  * @author Matthew Pocock
  */
object TestImplicitProblem {
  def main(args: Array[String]): Unit = {
    trait MyTypeclass[R[_]]

    implicit object StringImpl extends MyTypeclass[Const[String, ?]]
    implicit object IdImpl extends MyTypeclass[Id]

    implicit def productImpl[R1[_], R2[_]](implicit R1: MyTypeclass[R1], R2: MyTypeclass[R2])
    : MyTypeclass[Prod[R1, R2, ?]] = new MyTypeclass[Prod[R1, R2, ?]] {}

    // finds  this
    type si[T] = Const[String, T]
    val psiTpe = implicitly[MyTypeclass[si]]

    // finds both of these
    val si = implicitly[MyTypeclass[Const[String, ?]]]
    val idI = implicitly[MyTypeclass[Id]]

    // can't find this
    val psi = implicitly[MyTypeclass[Prod[Id, Const[String, ?], ?]]]

    // can't find this
    type psiT[T] = Prod[Id, Const[String, ?], T]
    val psiT = implicitly[MyTypeclass[psiT]]

    // nor this
    val psiPTpe = implicitly[MyTypeclass[Prod[Id, si, ?]]]

    // but can find this
    type psiTTpe[T] = Prod[Id, si, T]
    val psiTTpe = implicitly[MyTypeclass[psiTTpe]]
  }
}