map2.scala

      final override def map2[A, B, Z](fa: ParallelF[F, A], fb: ParallelF[F, B])(
          f: (A, B) => Z): ParallelF[F, Z] =
        ParallelF(
          F.uncancelable { poll =>
            for {
              fiberA <- F.start(ParallelF.value(fa))
              fiberB <- F.start(ParallelF.value(fb))

              // start a pair of supervisors to ensure that the opposite is canceled on error
              _ <- F start {
                fiberB.join flatMap {
                  case Outcome.Succeeded(_) => F.unit
                  case _ => fiberA.cancel
                }
              }

              _ <- F start {
                fiberA.join flatMap {
                  case Outcome.Succeeded(_) => F.unit
                  case _ => fiberB.cancel
                }
              }

              a <- F
                .onCancel(poll(fiberA.join), F.both(fiberA.cancel, fiberB.cancel).void)
                .flatMap[A] {
                  case Outcome.Succeeded(fa) =>
                    fa

                  case Outcome.Errored(e) =>
                    fiberB.cancel *> F.raiseError(e)

                  case Outcome.Canceled() =>
                    fiberB.cancel *> poll {
                      fiberB.join flatMap {
                        case Outcome.Succeeded(_) | Outcome.Canceled() =>
                          F.canceled *> F.never
                        case Outcome.Errored(e) =>
                          F.raiseError(e)
                      }
                    }
                }

              z <- F.onCancel(poll(fiberB.join), fiberB.cancel).flatMap[Z] {
                case Outcome.Succeeded(fb) =>
                  fb.map(b => f(a, b))

                case Outcome.Errored(e) =>
                  F.raiseError(e)

                case Outcome.Canceled() =>
                  poll {
                    fiberA.join flatMap {
                      case Outcome.Succeeded(_) | Outcome.Canceled() =>
                        F.canceled *> F.never
                      case Outcome.Errored(e) =>
                        F.raiseError(e)
                    }
                  }
              }
            } yield z
          }
        )