incorrect exhaustiveness warning in dotty build

[smarter]

Running sbt dotty-compiler-bootstrapped/compile we get:

[warn] -- [E028] Pattern Match Exhaustivity Warning: compiler/../out/bootstrap/dotty-compiler-bootstrapped/scala-0.1/src_managed/main/scala/tools/nsc/backend/jvm/BTypes.scala:337:35 
[warn] 337 |    def classOrArrayType: String = this match {
[warn]     |                                   ^^^^
[warn]     |                                   match may not be exhaustive.
[warn]     |                                   
[warn]     |                                   It would fail on: ClassBType(_)

But the match will not fail on ClassBType(_), the first case takes care of it:

    def classOrArrayType: String = this match {
      case ClassBType(internalName) => internalName
      case a: ArrayBType            => a.descriptor
    }

/cc @liufengyun

[liufengyun]

@smarter This is a problem in the backend, I already made a PR, could you review it?

lampepfl/scala#19

[liufengyun]

The problem is that it defines an extractor ClassBType which returns Option[String], thus it's taken as inexhaustive.

[smarter]

I see, but in this case the warning is incorrect, no? The misssing case is _: ClassBType, not ClassBType(_). It would also be good if the message contained a hint that you can make the unapply method irrefutable by return Some[X] instead of Option[X]

[liufengyun]

@smarter Yes, you are right, I'll update the warning message for extractors.

The suggestion is very good. We can add an explanation message: tell the programmers to either to have a wildcard or make the extractor irrefutable by returning Some[T].