runtime NoClassDefFoundError occurs on Scala 3.0.0, but not Scala 2.12, 2.13

[rtyley]

reproduction steps

Code for this issue can be found in https://github.com/rtyley/scala-2-vs-3-NoClassDefFoundError, relatively stripped down from where I actually encountered the issue when upgrading a library to Scala 3.0.0. The runtime error occurs when executing this code:

package object example { // Not using a package object allows Scala 3.0.0 to succeed
  def useCompressUtils(): Unit = {
    getClass // removing this allows Scala 3.0.0 to succeed.

    // Fails under Scala 3.0.0 with "java.lang.NoClassDefFoundError: com/madgag/compress/CompressUtil$"
    unzip(new ByteArrayInputStream(Array.emptyByteArray), createTempDirectory("example").toFile)
  }
}

When executing sbt test on the code, I get a java.lang.NoClassDefFoundError under Scala 3.0.0, but not under Scala 2.13.5 or 2.12.13. There is a Travis CI build to verify this:

The error given in sbt (an error report added to sbt with sbt/sbt#4602) suggests using a different ClassLoaderLayeringStrategy, however none of the available strategy settings (AllLibraryJars, Flat, ScalaLibrary) allow Scala 3.0.0 to succeed.

Scala 3.0.0 does pass if the code is modified in either of these ways:

• The call to getClass (originally used to load test resources in the library where this issue was found) is removed.
• The code is moved from a package object to a normal object.

problem

I would expect the code compiled under Scala 3.0.0 to work just as well as Scala 2.12 or 2.13, without throwing a NoClassDefFoundError!

[dwijnand]

Duplicate of scala/scala3#12249.

[smarter]

I don't think it's a duplicate, scala/scala3#12249 is about using the new export feature on Java code and that generating some incorrect code, whereas this issue is seemingly just about calling some Scala code and doesn't have anything to do with export. It does need to be minimized and re-opened in the dotty issue tracker though.

[SethTisue]

@smarter as an aside, it would be nice to get lampepfl/dotty relocated to scala/* before toooooo much longer, one reason being so GitHub would let us transfer issues between there and scala/bug

[smarter]

@SethTisue agreed, but definitely not something I want to tackle myself :).