JUnit tests defined in traits will crash the compiler with `test.Test0.initializationError failed`

[molikto]

minimized code

See this repo:

https://github.com/molikto/dotty-test

Compilation output

[error] Test test.Test0.initializationError failed: java.lang.Exception: Test class should have exactly one public constructor, took 0.002 sec
[error]     at com.novocode.junit.JUnitRunner$1.execute(JUnitRunner.java:124)
[error]     at sbt.TestRunner.runTest$1(TestFramework.scala:113)
[error]     at sbt.TestRunner.run(TestFramework.scala:124)
[error]     at sbt.TestFramework$$anon$2$$anonfun$$lessinit$greater$1.$anonfun$apply$1(TestFramework.scala:282)
[error]     at sbt.TestFramework$.sbt$TestFramework$$withContextLoader(TestFramework.scala:246)
[error]     at sbt.TestFramework$$anon$2$$anonfun$$lessinit$greater$1.apply(TestFramework.scala:282)
[error]     at sbt.TestFramework$$anon$2$$anonfun$$lessinit$greater$1.apply(TestFramework.scala:282)
[error]     at sbt.TestFunction.apply(TestFramework.scala:294)
[error]     at sbt.Tests$.$anonfun$toTask$1(Tests.scala:309)
[error]     at sbt.std.Transform$$anon$3.$anonfun$apply$2(System.scala:46)
[error]     at sbt.std.Transform$$anon$4.work(System.scala:67)
[error]     at sbt.Execute.$anonfun$submit$2(Execute.scala:269)
[error]     at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16)
[error]     at sbt.Execute.work(Execute.scala:278)
[error]     at sbt.Execute.$anonfun$submit$1(Execute.scala:269)
[error]     at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:178)
[error]     at sbt.CompletionService$$anon$2.call(CompletionService.scala:37)
[error]     at java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error]     at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
[error]     at java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error]     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
[error]     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
[error]     at java.lang.Thread.run(Thread.java:834)
[debug] Test test.Test0.initializationError finished, took 0.006 sec
[debug] Test run finished: 1 failed, 0 ignored, 1 total, 0.008s

expectation

Traits should not be traited as a test class, changing traits to abstract class works.

[OlivierBlanvillain]

I think you forgot to make your example repo public.

This doesn't look like a compiler crash, looking at the stack trace it's just JUnit complaining.

[sjrd]

It's not a compiler crash, but it's a bug somewhere in the Dotty compiler (probably the ExtractAPI logic) or in the sbt plugin for Dotty. My analysis is that the sbt test detection code somehow detects that this trait is a valid test suite, and consequently asks JUnit to run the tests in that test suite. JUnit then crashes because it's an interface, not a concrete class.

Since the sbt test detection is based on its inc.Analysis, which itself is based on the API exported by ExtractAPI, I assume something's wrong that logic. Unless the sbt plugin for dotty computes test detection in a different way, in which case that logic could also be at fault (but I would be surprised if it does).