unreachable code when using abstract case class pattern

[fommil]

In scalaz 7.3+ we're using this pattern to encode invariant ADTs

  sealed abstract class AsyncReturn[A]

  final case class Now[A](value: A)                  extends AsyncReturn[A]
  final case class MaybeLater[A](canceler: Canceler) extends AsyncReturn[A]

  sealed abstract case class Later[A] private ()     extends AsyncReturn[A]
  object Later {
    private[this] val value: Later[Nothing] = new Later[Nothing] {}
    def apply[A](): AsyncReturn[A]          = value.asInstanceOf[AsyncReturn[A]]
  }

Also discussed in #10660

However, we're now seeing false "unreachable code" errors, e.g. in
scalaz/ioeffect#9

I can't figure out how to minimise it. This IO code is dealing with low-level JVM details so it uses try and var a lot... it could be these interactions that are confusing the compiler.

Help appreciated to minimise.

In the meantime, being able to turn off the warning would be nice.

[adriaanm]

Without a repro it's hard to say what's going on. I see in that thread someone already suggested using silencer to turn off the warning.

[fommil]

There is a repro, just not a standalone file. It's a very small project.

Silencing the error is really a last resort thing. It seems to indicate a bug somewhere.

[hrhino]

Here's a reproduction:

Welcome to Scala 2.12.5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_161).
Type in expressions for evaluation. Or try :help.

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

  sealed abstract class AsyncReturn[A]

  final case class Now[A](value: A)                  extends AsyncReturn[A]

  sealed abstract case class Later[A] private ()     extends AsyncReturn[A]
  object Later {
    private[this] val value: Later[Nothing] = new Later[Nothing] {}
    def apply[A](): AsyncReturn[A]          = value.asInstanceOf[AsyncReturn[A]]
  }

object Test {
  def finish() = ()

  def test(ar: AsyncReturn[Int]) = try {
    ar match {
      case Now(value) => ()
      case Later()    => ()
    }
  } finally finish()
}

// Exiting paste mode, now interpreting.

<pastie>:17: warning: unreachable code
      case Later()    => ()
                         ^

It's possible that the pattern matcher considers Later never to match on account of the anonymous subclass being Later[Nothing] and knowing that ar's T is Int.

[hrhino]

That appears to be what it's doing:

checkableType(AsyncReturn[Int]): AsyncReturn[?]
checkableType(Now[Int]): Now[?]
uniqued const: (Now[?],Now[?])
checkableType(Later[Int]): Later[?]
uniqued const: (Later[?],Later[?])
removeVarEq vars: Set(V1)
enum class AsyncReturn sealed, subclasses: List(<$anon: Later[Nothing]>, class Now)
checkableType(Now[?]): Now[?]
enum sealed tp=AsyncReturn[Int], tpApprox=AsyncReturn[Int] as: List(Now[?])

[adriaanm]

Thank you, @hrhino! Really appreciate that.

So, that doesn't seem like a bug, then, right? Since the class is invariant.

[hrhino]

Changing this would probably break GADT checking, so probably not.

A workaround:

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

  sealed abstract class AsyncReturn[A]

  final case class Now[A](value: A)                  extends AsyncReturn[A]

  sealed abstract case class Later[A] private ()     extends AsyncReturn[A]
  object Later {
    private[this] final class Later_[+A] extends Later[A] {}
    private[this] val value = new Later_[Nothing]
    def Later[A](): AsyncReturn[A] = value.asInstanceOf[AsyncReturn[A]]
  }

// Exiting paste mode, now interpreting.


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

object Test {
  def finish() = ()

  def test(ar: AsyncReturn[Int]) = try {
    ar match {
      case Now(value) => ()
      case Later()    => ()
    }
  } finally finish()
}


// Exiting paste mode, now interpreting.


scala>

[adriaanm]

Nice!

[hrhino]

I'm going to mark as "won't fix" because this is probably an artifact of the GADT support. @adriaanm / @fommil please feel free to disagree.

[fommil]

it's like whack a mole