Please see the this Scala/Dotty issue.
This issue appears under Scala 3.3.6 and 3.7.0, with mill as well as sbt.
This repository is an sbt project. It compiles just fine, but if you try to run the test program...
> Test / run
an AbstractMethodError results:
[info] compiling 1 Scala source to /Users/swaldman/Documents/BaseFolders/development/playpen/inheritance-of-nested-traits-sbt/target/scala-3.3.6/classes ...
[info] compiling 1 Scala source to /Users/swaldman/Documents/BaseFolders/development/playpen/inheritance-of-nested-traits-sbt/target/scala-3.3.6/test-classes ...
[info] running logadapter.test.Test
Exception in thread "sbt-bg-threads-1" java.lang.AbstractMethodError: Receiver class logadapter.test.Test$ does not define or inherit an implementation of the resolved method 'abstract logadapter.AbstractApi logadapter$AbstractApi$SelfLogging$$$outer()' of interface logadapter.AbstractApi$SelfLogging.
at logadapter.AbstractApi$SelfLogging.adapter(core.scala:9)
at logadapter.AbstractApi$SelfLogging.adapter$(core.scala:8)
at logadapter.test.Test$.adapter$lzyINIT1(core.scala:3)
at logadapter.test.Test$.adapter(core.scala:3)
at logadapter.test.Test$.main(core.scala:5)
at logadapter.test.Test.main(core.scala)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at sbt.Run.invokeMain(Run.scala:135)
at sbt.Run.execute$1(Run.scala:85)
at sbt.Run.$anonfun$runWithLoader$5(Run.scala:112)
at sbt.Run$.executeSuccess(Run.scala:178)
at sbt.Run.runWithLoader(Run.scala:112)
at sbt.Defaults$.$anonfun$bgRunTask$6(Defaults.scala:2072)
at sbt.Defaults$.$anonfun$termWrapper$2(Defaults.scala:2011)
at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
at scala.util.Try$.apply(Try.scala:213)
at sbt.internal.BackgroundThreadPool$BackgroundRunnable.run(DefaultBackgroundJobService.scala:378)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
at java.base/java.lang.Thread.run(Thread.java:1583)
[success] Total time: 2 s, completed May 22, 2025, 11:30:46 PM
If you go to the root of the repository, and try:
$ mv src/test/scala/logadapter/test src/main/scala/logadapter
Then the program works fine:
> run
[info] running logadapter.test.Test
INFO [logadapter.test.Test$] Hello
[success] Total time: 0 s, completed May 22, 2025, 11:33:37 PM
Move it back...
$ mv src/main/scala/logadapter/test src/test/scala/logadapter
...and it is broken again.