Invalid implicit resolution attempt for higher kind

[carymrobbins]

Notice that scalac attempts to resolve an implicit for MonoidK[Option[String]] whereas it should be trying to resolve MonoidK[Option] -

Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131).
Type in expressions for evaluation. Or try :help.

scala> import shapeless._, cats._, cats.implicits._

scala>   object polyEmpty extends Poly0 {
     |     implicit def cases[F[_], V](implicit F: MonoidK[F]): ProductCase.Aux[HNil, F[V]] =
     |       at[F[V]](F.empty)
     |   }
defined object polyEmpty

scala> HList.fillWith[Option[String] :: HNil](polyEmpty)
<console>:22: this.polyEmpty.cases is not a valid implicit value for shapeless.poly.Case0.Aux[polyEmpty.type,Option[String]] because:
hasMatchingSymbol reported error: could not find implicit value for parameter F: cats.MonoidK[Option[String]]
       HList.fillWith[Option[String] :: HNil](polyEmpty)
                                             ^
<console>:22: hlist.this.FillWith.hconsFill is not a valid implicit value for shapeless.ops.hlist.FillWith[polyEmpty.type,Option[String] :: shapeless.HNil] because:
hasMatchingSymbol reported error: could not find implicit value for parameter hc: shapeless.poly.Case0.Aux[polyEmpty.type,Option[String]]
       HList.fillWith[Option[String] :: HNil](polyEmpty)
                                             ^
<console>:22: error: could not find implicit value for parameter fillWith: shapeless.ops.hlist.FillWith[polyEmpty.type,Option[String] :: shapeless.HNil]
       HList.fillWith[Option[String] :: HNil](polyEmpty)
                                             ^

[carymrobbins]

One workaround I came up with was to proxy the higher-kinded type class -

scala> :paste
// Entering paste mode (ctrl-D to finish)

  trait Field[A] {
    def combine(x: A, y: A): A
    def empty: A
  }

  object Field {
    implicit def fromMonoidK[F[_], A](implicit F: MonoidK[F]): Field[F[A]] = new Field[F[A]] {
      override def combine(x: F[A], y: F[A]): F[A] = F.combineK(x, y)
      override def empty: F[A]                     = F.empty
    }
  }

// Exiting paste mode, now interpreting.

defined trait Field
defined object Field

scala>   object polyEmpty extends Poly0 {
     |     implicit def cases[A](implicit F: Field[A]): Case0[A] = at[A](F.empty)
     |   }

scala> import shapeless.ops.hlist.FillWith

scala> val fill = implicitly[FillWith[polyEmpty.type, Option[String] :: HNil]]

scala> fill.apply
res1: fill.Out = None :: HNil

[joroKr21]

AppliedTypeVar should probably override some methods.

[joroKr21]

need minimization without shapeless for testing

[carymrobbins]

@joroKr21 Not sure if this helps, but I wrote a very minimal Generic type class that should be (nearly) a drop-in replacement. In this case we'd have to get rid of ZipWith and FillWith too, but I assume we can just have simpler, noop definition.

MiniGeneric.scala

[joroKr21]

Thanks! I forgot to mention it here, but I already added a test over at scala/scala#6573. All we need to trigger the bug is an Aux implicit (i.e. type parameters end up in a refinement) depending on a higher-kinded implicit.

[SethTisue]

@joroKr21 I want to assign you the ticket as a record of who fixed it, but you'll need to accept my invite to the contributors team first.