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Issue with type inference when pattern matching against GADT that contains higher-kinded parameter #10208

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scabug opened this Issue Feb 24, 2017 · 6 comments

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scabug commented Feb 24, 2017

This was noticed by @sellout when porting his work on slamdata/matryoshka#28 to matryoshka's master branch (which uses typelevel scala). He suggested it might be tied to the #9760 fix.

Here's the most minimal example I could come up with.

Compiles with LB Scala 2.11.8
Does not compile with TL Scala 2.11.8
Does not compile with 2.12.1 (using either LB or TL)

object Test1 {

  trait ~>[A[_], B[_]] {
    def apply[I](fa: A[I]): B[I]
  }

  // HIGHER KINDED GADT

  sealed trait GADTK[A[_], I]
  final case class MemberK[A[_]](i: Int) extends GADTK[A, Int]

  def doesNotCompile[A[_]]: (GADTK[A, ?] ~> GADTK[A, ?]) =
    new (GADTK[A, ?] ~> GADTK[A, ?]) {
      def apply[I](v: GADTK[A, I]): GADTK[A, I] = v match {
        case MemberK(i) => MemberK(i)
      }
    }

  class CompilesFine[A[_]] extends (GADTK[A, ?] ~> GADTK[A, ?]) {
    def apply[I](v: GADTK[A, I]): GADTK[A, I] = v match {
      case MemberK(i) => MemberK(i)
    }
  }

  // SIMPLE GADT

  sealed trait GADT[A, I]
  case class Member[A, I](i: Int) extends GADT[A, Int]

  def compilesFine[A]: (GADT[A, ?] ~> GADT[A, ?]) =
    new (GADT[A, ?] ~> GADT[A, ?]) {
      def apply[I](v: GADT[A, I]): GADT[A, I] = v match {
        case Member(i) => Member(i)
      }
    }
}
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scabug commented Feb 24, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10208?orig=1
Reporter: Olivier Mélois (Baccata)
Affected Versions: 2.12.0
See #9760

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scabug commented Feb 24, 2017

@milessabin said:
If this is a consequence of the fix for #9760 then it will most likely be a regression in 2.11.9 as well.

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scabug commented Feb 24, 2017

Greg Pfeil (sellout) said:
Yeah, just verified that reverting the #9760 fix allows this to compile. Now to understand #9760

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scabug commented Feb 24, 2017

Greg Pfeil (sellout) said:
Ok, so I think I’ve narrowed it down. While checking MemberK[Any] <:< GADTK[?A, ?I], it fails on Any <:< ?A. Note that both the type parameter to MemberK and ?A are of kind * -> *, while Any isn’t.

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scabug commented Feb 26, 2017

Olivier Mélois (Baccata) said:
Any seems to come from the resolution of A to ?A, specifically this line : https://github.com/scala/scala/blob/98a7d4ce0234dc1c4b080ecaeed9ced09d39578a/src/reflect/scala/reflect/internal/tpe/TypeConstraints.scala#L254

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scabug commented Feb 26, 2017

Greg Pfeil (sellout) said:
I submitted a patch the other night – I guess it would be good to link it here. scala/scala#5744

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