PatternMatchingHack.scala

sealed trait Foo[F[_], A]
case class Bar[F[_], A](f: F[A]) extends Foo[F, A]
case class Baz[F[_], A](f: F[A]) extends Foo[F, String]

sealed trait Qux[F[_], A] {
  def value: F[A]
  def stuff: F[String]
}

trait Arr[F[_], G[_]] {
  def apply[A](f: F[A]): G[A]
}

object Test {
  /**
   * Compiles on 2.11.8 but not 2.11.11 or 2.12.0-2.
   */
  def test1[F[_], G[x] <: Qux[F, x]]: Arr[({ type L[x] = Foo[G, x] })#L, F] =
    new Arr[({ type L[x] = Foo[G, x] })#L, F] {
      def apply[A](foo: Foo[G, A]): F[A] = foo match {
        case Bar(f) => f.value
        case Baz(f) => f.stuff
      }
  }

  class Hack[F[_], G[x] <: Qux[F, x]] {
    def apply[A](foo: Foo[G, A]): F[A] = foo match {
      case Bar(f) => f.value
      case Baz(f) => f.stuff
    }
  }

  /**
   * Compiles on 2.11.8, 2.11.11, and 2.12.0-2.
   */
  def test2[F[_], G[x] <: Qux[F, x]]: Arr[({ type L[x] = Foo[G, x] })#L, F] =
    new Arr[({ type L[x] = Foo[G, x] })#L, F] {
      def apply[A](foo: Foo[G, A]): F[A] = new Hack[F, G].apply(foo)
  }
}