Questionable warnings while matching the opaque type IArray

[devlaam]

Observation

Compile and run the code with Scala 3.1.1: Scastie link

object MatchTest :
  val a: Array[String] = Array()
  val b: Array[String] = Array("x")
  val c: Array[String] = Array("a","b")

  def test(x: Array[String]) = x match
    case IArray()          => println("Empty")
    case IArray(i: String) => println("Caught")
    case _                 => println("Failure")

  def main(args: Array[String]): Unit =
    test(a)
    test(b)
    test(c)

Result of compilation:

[warn] -- Unchecked Warning: ...src/main/scala/Main.scala:7:9 
[warn] 7 |    case IArray()          => println("Empty")
[warn]   |         ^
[warn]   |         the type test for IArray[Any] cannot be checked at runtime
[warn] -- [E030] Match case Unreachable Warning: ...src/main/scala/Main.scala:8:9 
[warn] 8 |    case IArray(i: String) => println("Caught")
[warn]   |         ^^^^^^^^^^^^^^^^^
[warn]   |         Unreachable case

Result of running:

Empty
Caught
Failure

Expectation

Based on the runtime result i would say both warnings were not expected. The question remains if the opaque type IArray should match a normal Array at all. Probably it would do no harm, but it is somewhat counter intuitive, since IArray is derived from Array and not the other way around.

Discussion

• I am uncertain if this qualifies as compiler bug or not.
• This showcase can be used as a workaround for the bug Missed match on Array(1L) in Scala 3.1.1
• If you compile with the option -source:future, Scastie link, the Unreachable Warning is replaced by an Unmatchable Type Warning, which indeed is more adequate. The question if an immutable type should be unmatchable should be discussed an other time.

[odersky]

I think this is as expected.

[nicolasstucki]

Should it be an unchecked warning instead of unreachable?

Because it is unreachable as long as it can be type tested. Maybe we should emit both warnings.