Misleading `Unreachable case` report in `ensuring`. Bug or feature?

[martinberger]

Compiler version

scalac 3.6.3

sbt: 1.10.7 with the following options in build.sbt:

scalacOptions ++= Seq(
  "-language:strictEquality", 
  "-deprecation",  
  "-explain",  
  "-explain-types",  
  "-feature", 
  "-indent", 
  "-new-syntax",  
  "-print-lines", 
  "-unchecked",  
  "-Xkind-projector", 
  "-Xmigration", 
  "-source:3.7"  
)

Minimized example

def f(n: Int): Option[Int] = {
    if n == 0 then None else Some(n)
} ensuring (
  _ match {
      case Some(i) => i != 0
      case None    => true
  }
)

Output Error/Warning message

sbt:Martin Berger> ~compile
[info] compiling 12 Scala sources to  ...
[warn] -- [E030] Match case Unreachable Warning: .../Main.scala:45:9 
[warn] 45 |    case None      => true
[warn]    |         ^^^^
[warn]    |         Unreachable case
[warn] one warning found

Note: I get this warning only when compiling with sbt. If I compile with scalac -Wall there is no warning!

Why this Error/Warning was not helpful

The message Unreachable case is unhelpful because the None case in the ensuring construct is in fact reachable. This can easily be seen by replacing case None => true with case None => { assert(false); true } and call f(0).

Suggested improvement

I think this is a bug in the type-inferencer. I did not file a bug report, because I wasn't sure if I had not overlooked some subtle feature of the typing system.

[som-snytt]

I could not reproduce it. Are there sbt plugins involved?

Your example says compiling 12 Scala sources, so there are at least eleven more sources involved.

You could link to a sample project.

[martinberger]

There are no sbt plugins involved. The additional sources should not be relevant, but when I tried in an empty sbt project just now, I could also not reproduce the behaviour. Let me try to minimise my whole 12-file project and see what is causing this behaviour.