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

[Baccata]

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

[milessabin]

This was merged in LBS 2.12.0, so the Scala JIRA is the place for this report.