0.9.8→0.9.9 regression? but SSC(C)E triggers on earlier versions too

I have a new build failure on my project which traces to an Update dotty-cps-async to 0.9.9 commit. I have tried to minimise the failing code to the shortest possible example, but after minimisation it seems to trigger the same problem in older versions too.

I will give the minimised example here:

import cps.*
import cps.monads.FutureAsyncMonad

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

class TestCase:
  val objs = Seq(
    new AnyRef:
      def curry(v: Any)(w: Any): Unit = ()

      def bad_func(): Future[Unit] =
        async {
          for message <- Nil.toSeq do
            curry(Nil.toSeq)
          ()
        }
  )

This code has quickly turned into nonsense, but nonetheless it triggers the following failure:

java.lang.Exception: unexpected tree: closure($anonfun)
dotty.tools.dotc.transform.init.Semantic$.cases(Semantic.scala:1271)
dotty.tools.dotc.transform.init.Semantic$.eval$$anonfun$1(Semantic.scala:1071)
dotty.tools.dotc.transform.init.Semantic$Cache$Cache.assume(Semantic.scala:314)
dotty.tools.dotc.transform.init.Semantic$.eval(Semantic.scala:1071)
dotty.tools.dotc.transform.init.Semantic$.cases(Semantic.scala:1195)
dotty.tools.dotc.transform.init.Semantic$.eval$$anonfun$1(Semantic.scala:1071)
dotty.tools.dotc.transform.init.Semantic$Cache$Cache.assume(Semantic.scala:314)
dotty.tools.dotc.transform.init.Semantic$.eval(Semantic.scala:1071)
dotty.tools.dotc.transform.init.Semantic$.$anonfun$22(Semantic.scala:903)
dotty.tools.dotc.transform.init.Semantic$.promote(Semantic.scala:903)
dotty.tools.dotc.transform.init.Semantic$ArgInfo.promote(Semantic.scala:1051)
[...]

Some interesting things break this example and cause the code to compile properly again:

• Turning the val into a def
• Wrapping the val in a function
• Not wrapping the object in a Seq
• Using a sequence which isn’t generated through a function call on another object (i.e. Nil.toSeq causes an error, but Nil itself doesn’t cause the error. One could replace it with something like Some(Seq(1, 2, 3)).get and still encounter an error)
• Moving the anonymous class to an outer class (but inner classes/objects still fail)

This appears to be a confluence of many factors, to a somewhat surprising degree of complexity.

The original project at the broken commit is here: https://github.com/ScoreUnder/canti-bot/tree/cb179d537c0494ddddb4dfd24904f75aae30611c

The offending function on which the example is based is here: https://github.com/ScoreUnder/canti-bot/blob/cb179d537c0494ddddb4dfd24904f75aae30611c/src/main/scala/score/discord/canti/functionality/Spoilers.scala#L74-L90

If you edit the build.sbt in that project to lower the version of dotty-cps-async from 0.9.9 to 0.9.8, it will compile properly again.

[rssh]

preparing a next release, will write here, when it will be available.

[ScoreUnder]

All good from my end, thank you!