SIP-47 Clause Interleaving + Java = bad class file error

[SethTisue]

Discussed in #21195

^{Originally posted by xuwei-k July 15, 2024}
Is this expected behavior? or we should/can fix this error? 🤔

A.scala

package example

class A {
  def f[B](b: B)[C](c: C): (B, C) = (b, c)
}

build.sbt

scalaVersion := "3.6.0-RC1-bin-20240712-97a711c-NIGHTLY"

X.java

package j;

import example.A;

class X {
  A a = new A();
}

sbt compile

[error] example-project-dir/X.java:3:1: cannot access example.A
[error]   bad class file: example-project-dir/target/scala-3.6.0-RC1-bin-20240712-97a711c-NIGHTLY/classes/example/A.class
[error]     undeclared type variable: C
[error]     Please remove or make sure it appears in the correct subdirectory of the classpath.

javap -v A.class

  public <B extends java.lang.Object> scala.Tuple2<B, C> f(B);
    descriptor: (Ljava/lang/Object;Ljava/lang/Object;)Lscala/Tuple2;

getGenericParameterTypes and getGenericReturnType

Welcome to Scala 3.6.0-RC1-bin-20240712-97a711c-NIGHTLY-git-97a711c (11.0.23, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                             
scala> val f = classOf[example.A].getMethods.find(_.getName == "f").get
val f: java.lang.reflect.Method = public scala.Tuple2 example.A.f(java.lang.Object,java.lang.Object)
                                                                                                                                             
scala> f.getGenericParameterTypes
val res0: Array[java.lang.reflect.Type] = Array(B)
                                                                                                                                             
scala> f.getGenericReturnType
java.lang.NullPointerException
  at java.base/sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl.toString(ParameterizedTypeImpl.java:229)
  at scala.runtime.ScalaRunTime$.inner$1(ScalaRunTime.scala:258)
  at scala.runtime.ScalaRunTime$.stringOf(ScalaRunTime.scala:263)
  at scala.runtime.ScalaRunTime.stringOf(ScalaRunTime.scala)
  at jdk.internal.reflect.GeneratedMethodAccessor5.invoke(Unknown Source)
  at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.base/java.lang.reflect.Method.invoke(Method.java:566)
  at dotty.tools.repl.Rendering.poorStringOf$1$$anonfun$1(Rendering.scala:64)
  at dotty.tools.repl.Rendering.poorStringOf$1$$anonfun$adapted$1(Rendering.scala:64)
  at dotty.tools.repl.Rendering.stringOfMaybeTruncated$1(Rendering.scala:68)
  at dotty.tools.repl.Rendering.classLoader$$anonfun$1(Rendering.scala:77)
  at dotty.tools.repl.Rendering.classLoader$$anonfun$adapted$1(Rendering.scala:76)
  at dotty.tools.repl.Rendering.replStringOf(Rendering.scala:105)
  at dotty.tools.repl.Rendering.valueOf$$anonfun$2(Rendering.scala:123)
  at scala.Option.map(Option.scala:242)
  at dotty.tools.repl.Rendering.valueOf(Rendering.scala:123)
  at dotty.tools.repl.Rendering.renderVal(Rendering.scala:160)
  at dotty.tools.repl.ReplDriver.$anonfun$7(ReplDriver.scala:402)
  at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
  at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
  at scala.collection.immutable.List.foreach(List.scala:334)
  at dotty.tools.repl.ReplDriver.extractAndFormatMembers$1(ReplDriver.scala:402)
  at dotty.tools.repl.ReplDriver.renderDefinitions$$anonfun$2(ReplDriver.scala:440)
  at scala.Option.map(Option.scala:242)
  at dotty.tools.repl.ReplDriver.renderDefinitions(ReplDriver.scala:439)
  at dotty.tools.repl.ReplDriver.compile$$anonfun$2(ReplDriver.scala:342)
  at scala.util.Either.fold(Either.scala:197)
  at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:324)
  at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:283)
  at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:196)
  at dotty.tools.repl.ReplDriver.runUntilQuit$$anonfun$1(ReplDriver.scala:199)
  at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:238)
  at dotty.tools.repl.ReplDriver.runBody$$anonfun$1(ReplDriver.scala:212)
  at dotty.tools.runner.ScalaClassLoader$.asContext(ScalaClassLoader.scala:80)
  at dotty.tools.repl.ReplDriver.runBody(ReplDriver.scala:212)
  at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:199)
  at xsbt.ConsoleInterface.run(ConsoleInterface.java:52)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.base/java.lang.reflect.Method.invoke(Method.java:566)
  at sbt.internal.inc.AnalyzingCompiler.invoke(AnalyzingCompiler.scala:329)
  at sbt.internal.inc.AnalyzingCompiler.console(AnalyzingCompiler.scala:233)
  at sbt.Console.console0$1(Console.scala:65)
  at sbt.Console.$anonfun$apply$5(Console.scala:75)
  at sbt.Run$.executeSuccess(Run.scala:187)
  at sbt.Console.$anonfun$apply$4(Console.scala:75)
  at sbt.internal.util.Terminal.withRawInput(Terminal.scala:146)
  at sbt.internal.util.Terminal.withRawInput$(Terminal.scala:144)
  at sbt.internal.util.Terminal$ProxyTerminal$.withRawInput(Terminal.scala:424)
  at sbt.Console.$anonfun$apply$3(Console.scala:75)
  at sbt.internal.util.Terminal$TerminalImpl.withRawOutput(Terminal.scala:1028)
  at sbt.internal.util.Terminal$ProxyTerminal$.withRawOutput(Terminal.scala:463)
  at sbt.Console.apply(Console.scala:72)
  at sbt.Console.apply(Console.scala:50)
  at sbt.Console.apply(Console.scala:42)
  at sbt.Defaults$.$anonfun$consoleTask$1(Defaults.scala:2287)
  at sbt.Defaults$.$anonfun$consoleTask$1$adapted(Defaults.scala:2273)
  at scala.Function1.$anonfun$compose$1(Function1.scala:49)
  at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:63)
  at sbt.std.Transform$$anon$4.work(Transform.scala:69)
  at sbt.Execute.$anonfun$submit$2(Execute.scala:283)
  at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:24)
  at sbt.Execute.work(Execute.scala:292)
  at sbt.Execute.$anonfun$submit$1(Execute.scala:283)
  at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
  at sbt.CompletionService$$anon$2.call(CompletionService.scala:65)
  at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
  at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
  at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
  at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
  at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
  at java.base/java.lang.Thread.run(Thread.java:829)
```</div>

[SethTisue]

at #20861 (comment) , @sjrd wrote:

I don't think [this is a blocker for SIP-47]. It only affects new code combined with Java code. It cannot break anyone immediately. We should fix it, though, for sure, but not a blocker IMO.