`is not a trait` error when extends Java annotation

[xuwei-k]

Compiler version

3.0.1-RC1, 3.0.2-RC1-bin-20210615-a8bbc0e-NIGHTLY

Minimized code

A.scala

package example

class A

Named.java

package example;

public @interface Named {}

abstract class NamedImpl implements Named {}

Output

[error] 5 |abstract class NamedImpl implements Named {}
[error]   |                                    ^^^^^
[error]   |                                    class Named is not a trait
[error] one error found

Expectation

compile success because javac Named.java success.

Note

Could not build playframework due to this issue.

https://github.com/playframework/playframework/blob/d1f59e95892c69b1bd7e03d63997bb8c54a8de8a/core/play/src/main/java/play/inject/NamedImpl.java#L7-L17

[romanowski]

3.0 is also affected.

[odersky]

It's most likely a problem in JavaParser.

[smarter]

We intentionally pretend that Java annotations are classes, this is done to make them closer to Scala annotations and avoid special-casing, but it does mean that things can go wrong sometimes. Scala 2 used to do the same but switched to representing them as traits a while ago (#5690 (comment) has the history) which carries its own set of challenges, and even if we changed our representation now, we'd still need to support the old representation coming from tasty files compiled with scala 3.0. In brief, it's a mess.

[xuwei-k]

another example

build.sbt

libraryDependencies += "com.typesafe.play" %% "play" % "2.8.8" cross CrossVersion.for3Use2_13

scalaVersion := "3.0.1"

Main.scala

object Main {
  new play.inject.NamedImpl("")
}

log

[info] compiling 1 Scala source to /Users/kenji/scala-3-issue-12840/target/scala-3.0.1/classes ...
Error while emitting Main.scala
[info] exception occurred while compiling /Users/kenji/scala-3-issue-12840/Main.scala
java.lang.AssertionError: assertion failed: Invalid interfaces in Lplay/inject/NamedImpl;: List(Ljavax/inject/Named;, Ljava/io/Serializable;) while compiling /Users/kenji/scala-3-issue-12840/Main.scala
[error] ## Exception when compiling 1 sources to /Users/kenji/scala-3-issue-12840/target/scala-3.0.1/classes
[error] java.lang.AssertionError: assertion failed: Invalid interfaces in Lplay/inject/NamedImpl;: List(Ljavax/inject/Named;, Ljava/io/Serializable;)
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error] dotty.tools.backend.jvm.BTypes$ClassBType.checkInfoConsistency(BTypes.scala:623)
[error] dotty.tools.backend.jvm.BTypes$ClassBType.info_$eq(BTypes.scala:600)
[error] dotty.tools.backend.jvm.BTypesFromSymbols.setClassInfo(BTypesFromSymbols.scala:200)
[error] dotty.tools.backend.jvm.BTypesFromSymbols.classBTypeFromSymbol$$anonfun$1(BTypesFromSymbols.scala:93)
[error] scala.collection.mutable.HashMap.getOrElse(HashMap.scala:436)
[error] dotty.tools.backend.jvm.BTypesFromSymbols.classBTypeFromSymbol(BTypesFromSymbols.scala:94)
[error] dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.getClassBTypeAndRegisterInnerClass(BCodeHelpers.scala:270)
[error] dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.getClassBTypeAndRegisterInnerClass$(BCodeHelpers.scala:210)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.getClassBTypeAndRegisterInnerClass(BCodeSkelBuilder.scala:63)
[error] dotty.tools.backend.jvm.BCodeHelpers.primitiveOrClassToBType$2$$anonfun$1(BCodeHelpers.scala:808)
[error] scala.collection.immutable.HashMap.getOrElse(HashMap.scala:683)
[error] dotty.tools.backend.jvm.BCodeHelpers.primitiveOrClassToBType$1(BCodeHelpers.scala:808)
[error] dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$typeToTypeKind(BCodeHelpers.scala:827)
[error] dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.toTypeKind(BCodeHelpers.scala:299)
[error] dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.toTypeKind$(BCodeHelpers.scala:210)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.toTypeKind(BCodeSkelBuilder.scala:63)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genApply(BCodeBodyBuilder.scala:712)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:346)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genStat(BCodeBodyBuilder.scala:96)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genBlock$$anonfun$1(BCodeBodyBuilder.scala:895)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genBlock(BCodeBodyBuilder.scala:895)
[error] dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:417)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.emitNormalMethodBody$1(BCodeSkelBuilder.scala:765)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:800)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:608)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen$$anonfun$1(BCodeSkelBuilder.scala:614)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:614)
[error] dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:229)
[error] dotty.tools.backend.jvm.GenBCodePipeline$Worker1.visit(GenBCode.scala:229)
[error] dotty.tools.backend.jvm.GenBCodePipeline$Worker1.run(GenBCode.scala:194)
[error] dotty.tools.backend.jvm.GenBCodePipeline.buildAndSendToDisk(GenBCode.scala:529)
[error] dotty.tools.backend.jvm.GenBCodePipeline.run(GenBCode.scala:495)
[error] dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:60)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:303)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:304)
[error] dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:64)
[error] dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$5(Run.scala:216)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:224)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:231)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:166)
[error] dotty.tools.dotc.Run.compile(Run.scala:150)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
[error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
[error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
[error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:186)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:241)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:176)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:157)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:157)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:204)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:174)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:172)
[error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:457)
[error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
[error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:261)
[error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:412)
[error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:499)
[error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:399)
[error] sbt.internal.inc.Incremental$.apply(Incremental.scala:166)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
[error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
[error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
[error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2357)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2314)
[error] sbt.internal.io.Retry$.apply(Retry.scala:46)
[error] sbt.internal.io.Retry$.apply(Retry.scala:28)
[error] sbt.internal.io.Retry$.apply(Retry.scala:23)
[error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:31)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2310)
[error] scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] sbt.Execute.work(Execute.scala:291)
[error] sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[error] java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[error] java.lang.Thread.run(Thread.java:748)

[dotordogh]

We are also running into this issue when trying to cross-build twitter/util's util-reflect. An example is the Widget and it's implementation. We get the same error:

[error] -- [E104] Syntax Error: /Users/dordogh/workspace/source/util/util-reflect/src/test/java/com/twitter/util/reflect/WidgetImpl.java:6:28
[error] 6 |class WidgetImpl implements Widget, Serializable {
[error]   |                            ^^^^^^
[error]   |                            class Widget is not a trait

This is running with scala 3.0.2-RC1.