3.3.0-RC4 -Wunused:all compiler exception

[ornicar]

When compiling lila with:

• scala 3.3.0-RC4
• -Wunused:all flag
• openjdk version "17.0.6" 2023-01-17
• sbt 1.8.2

we get the following exception:

java.lang.AssertionError: NoDenotation.owner while running checkUnusedPostInlining on lila/modules/tree/src/main/Eval.scala
[info] exception occurred while compiling lila/modules/tree/src/main/Eval.scala, lila/modules/tree/src/main/package.scala, lila/modules/tree/src/main/tree.scala
java.lang.AssertionError: NoDenotation.owner while compiling lila/modules/tree/src/main/Eval.scala, lila/modules/tree/src/main/package.scala, lila/modules/tree/src/main/tree.scala
[error] ## Exception when compiling 3 sources to lila/modules/tree/target/scala-3.3.0-RC4/classes
[error] java.lang.AssertionError: NoDenotation.owner

Here's the full compiler output.

It works if we remove the -Wunused:all flag from the build settings, or if we downgrade scala to 3.3.0-RC3.

How to reproduce:

git clone https://github.com/lichess-org/lila.git --branch scala-3.3.0-RC4
cd lila
sbt compile

[ornicar]

It can also be reproduced with lila-ws, a much smaller project.

git clone https://github.com/lichess-org/lila-ws.git --branch Wunused-all
cd lila-ws
sbt compile

[WojciechMazur]

Also present in 3 Open CB projects:

Project	Version	Build URL	Notes
cchantep/acolyte	1.2.6	Open CB logs	Contains other compilation errors (scaladoc)
reactivemongo/reactivemongo-bson	1.1.0-RC9	Open CB logs
reactivemongo/reactivemongo-play-json	1.1.0-play29-RC9	Open CB logs

[KacperFKorban]

The common factor is reactivemongo-bson-api.
Partial minimization with just reactivemongo-bson-api as dependency:

//> using scala "3.3.0-RC4-bin-SNAPSHOT"
//> using options "-Wunused:all"
//> using lib "org.reactivemongo::reactivemongo-bson-api:1.1.0-RC9"

import reactivemongo.api.bson.*

case class Eval()

given BSONDocumentHandler[Eval] = Macros.handler

[Kordyjan]

We have identified the symbol that is missing a denotation, and this is definitely some weird bug. It shouldn't cause further problems but still, we need to investigate it.

The investigation will take place under #17346. In the meantime, #17316 fixed the symptoms.