feature request: data flow based exhaustive checker

[He-Pin]

  def futureSource[T, M](futureSource: Future[Source[T, M]]): Source[T, Future[M]] = {
    val maybeValue = futureSource.value
    if (maybeValue eq None) {
      fromGraph(new FutureFlattenSource(futureSource))
    } else maybeValue match {
      case Some(scala.util.Success(null)) =>
        val exception = new NullPointerException("futureSource completed with null")
        Source.failed(exception).mapMaterializedValue(_ => Future.failed[M](exception))
      case Some(scala.util.Success(source)) => source.mapMaterializedValue(Future.successful)
      case Some(scala.util.Failure(ex))     => Source.failed[T](ex).mapMaterializedValue(_ => Future.failed[M](ex))
    }
  }

I have the current code, but the compiler say it will fails

[error] /Users/hepin/IdeaProjects/pekko/stream/src/main/scala/org/apache/pekko/stream/scaladsl/Source.scala:599:12: match may not be exhaustive.
[error] It would fail on the following input: None
[error] Applicable -Wconf / @nowarn filters for this fatal warning: msg=<part of the message>, cat=other-match-analysis, site=org.apache.pekko.stream.scaladsl.Source.futureSource
[error]     } else maybeValue match {
[error]            ^
[error] two errors found
[error] (stream / Compile / compileIncremental) Compilation failed

[sjrd]

You can integrate your None test into the match, and then you'll get exhaustivity.

[som-snytt]

if x == None || flag then f() else x match case Some(...) =>
is not convenient to express.
x match { case None => f() case _ if flag => f() case ...
unlike submatches for "match this and if match that".
Does a guard still mean no exhaustivity?
Just speculating because I have house chores to procrastinate.
I guess sub-cases are still a secret.
https://dotty.epfl.ch/docs/reference/experimental/sub-cases.html

[He-Pin]

Yes, I just found that too, should rewrite all in pattern matching

[som-snytt]

I'd forgotten about scala/improvement-proposals#37 "uncluttering abuse of match". I could reopen it with new syntax for "boolean alternative". It's like a criminal case that can be "retried" when new evidence comes to light. (Just kidding, mostly.)