Switch to Scala 3 - fails (due to ClassLoaderLayeringStrategy?)

[rtyley]

Switching to Scala 3 (from Scala 2.13.16) fails:

[error] Test suite com.madgag.git.ReachableBlobSpec failed with java.lang.NoClassDefFoundError: com/madgag/compress/CompressUtil$.
[error] This may be due to the ClassLoaderLayeringStrategy (ScalaLibrary) used by your task.
[error] To improve performance and reduce memory, sbt attempts to cache the class loaders used to load the project dependencies.
[error] The project class files are loaded in a separate class loader that is created for each test run.
[error] The test class loader accesses the project dependency classes using the cached project dependency classloader.
[error] With this approach, class loading may fail under the following conditions:
[error] 
[error]  * Dependencies use reflection to access classes in your project's classpath.
[error]    Java serialization/deserialization may cause this.
[error]  * An open package is accessed across layers. If the project's classes access or extend
[error]    jvm package private classes defined in a project dependency, it may cause an IllegalAccessError
[error]    because the jvm enforces package private at the classloader level.
[error] 
[error] These issues, along with others that were not enumerated above, may be resolved by changing the class loader layering strategy.
[error] The Flat and ScalaLibrary strategies bundle the full project classpath in the same class loader.
[error] To use one of these strategies, set the ClassLoaderLayeringStrategy key
[error] in your configuration, for example:
[error] 
[error] set scala-git / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.ScalaLibrary
[error] set scala-git / Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat
[error] 
[error] See ClassLoaderLayeringStrategy.scala for the full list of options.

Unfortunately, even if we follow the advice of the setting the ClassLoaderLayeringStrategy, the build still fails:

• Use ClassLoaderLayeringStrategy.ScalaLibrary #15
• Use ClassLoaderLayeringStrategy.Flat #16

Forking also fails:

• Try forking tests #17

See also:

• Add cache of layered classloaders for test and run tasks sbt/sbt#4476