Support main methods from traits in Scala 3.4 (200USD Bounty)

[UnaiUribarri-TomTom]

---

From the maintainer Li Haoyi: I'm putting a 200USD bounty on this issue, payable by bank transfer on a merged PR resolving this issue, including a unit test to avoid regression. The issue is straightforward to reproduce.

---

My Main object has grown too large and I want to split it. I tried to create a trait for each entry point and then making the Main object extends all those traits. But in such case, mainargs does not find any of the @main annotated entry points in any of the traits, while it worked flawlessly when all those methods were together in the same Main object.

trait CommandList {
  @main
  def list(@arg v: String): Unit = ???
}

trait CommandCopy {
  @main
  def copy(@arg from: String, @arg to: String): Unit = ???
}

object Main extends CommandList with CommandCopy {
  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

Is there any way to support this use case? Without having to duplicate the entry point in all of them, of course.

[lihaoyi]

This seems reasonable. I thought it would work already, but I've never tried and I guess if you're opening this ticket it doesn't. Should be implementable

[lihaoyi]

Seems this works in 2.13 but not in 3.4

[arainko]

I'll try to get this one working 😸

[arainko]

For an example like this one:

trait CommandCopy {
  @main
  def copy(@arg from: String, @arg to: String): Unit = ???
}

class Joined extends  CommandCopy {
  @main
  def test(@arg from: String, @arg to: String): Unit = ???

  val parser = ParserForMethods(this)
}

The issue manifests itself as a compiler crash:

[error] ## Exception when compiling 23 sources to /home/aleksander/repos/mainargs/out/mainargs/jvm/3.3.1/test/compile.dest/classes
[error] java.lang.AssertionError: assertion failed: missing outer accessor in class Joined
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error] dotty.tools.dotc.transform.ExplicitOuter$.dotty$tools$dotc$transform$ExplicitOuter$$$outerParamAccessor(ExplicitOuter.scala:237)
[error] dotty.tools.dotc.transform.ExplicitOuter$OuterOps$.loop$1(ExplicitOuter.scala:474)
[error] dotty.tools.dotc.transform.ExplicitOuter$OuterOps$.path$extension(ExplicitOuter.scala:483)

The code that gets generated looks like this:

(new ParserForMethods[Joined](
  MethodMains.apply[Joined](
    List.apply[MainData[Any, Joined]](
      MainData.create[Any, Joined](
        "copy",
        new main(main.$lessinit$greater$default$1, main.$lessinit$greater$default$2),
        List.apply[ArgSig](
          ArgSig.create[String, Joined](
            "from",
            new arg(arg.$lessinit$greater$default$1, arg.$lessinit$greater$default$2, arg.$lessinit$greater$default$3, arg.$lessinit$greater$default$4, arg.$lessinit$greater$default$5, arg.$lessinit$greater$default$6),
            None
          )(StringRead), 
          ArgSig.create[String, Joined](
            "to",
            new arg(arg.$lessinit$greater$default$1, arg.$lessinit$greater$default$2, arg.$lessinit$greater$default$3, arg.$lessinit$greater$default$4, arg.$lessinit$greater$default$5, arg.$lessinit$greater$default$6),
            None
          )(StringRead)),
        ((b: Joined, params: [A >: Nothing <: Any] =>> Seq[A][Any]) =>
          CommandCopy.this.copy(
            Seq.apply[Seq[Any]](params).apply(0).apply(0).asInstanceOf[String],
            Seq.apply[Seq[Any]](params).apply(0).apply(1).asInstanceOf[String]
          )
        )),
      MainData.create[Any, Joined](
        "test",
        new main(main.$lessinit$greater$default$1, main.$lessinit$greater$default$2),
        List.apply[ArgSig](
          ArgSig.create[String, Joined](
            "from",
            new arg(arg.$lessinit$greater$default$1, arg.$lessinit$greater$default$2, arg.$lessinit$greater$default$3, arg.$lessinit$greater$default$4, arg.$lessinit$greater$default$5, arg.$lessinit$greater$default$6),
            None
          )(StringRead),
        ArgSig.create[String, Joined](
          "to",
          new arg(arg.$lessinit$greater$default$1, arg.$lessinit$greater$default$2, arg.$lessinit$greater$default$3, arg.$lessinit$greater$default$4, arg.$lessinit$greater$default$5, arg.$lessinit$greater$default$6),
          None
        )(StringRead)),
        ((`b₂`: Joined, `params₂`: [A >: Nothing <: Any] =>> Seq[A][Any]) =>
          Joined.this.test(
            Seq.apply[Seq[Any]](`params₂`).apply(0).apply(0).asInstanceOf[String],
            Seq.apply[Seq[Any]](`params₂`).apply(0).apply(1).asInstanceOf[String]
          )
        ))), (() => Joined.this))
    ): ParserForMethods[Joined])

The issue itself stems from this:

((b: Joined, params: [A >: Nothing <: Any] =>> Seq[A][Any]) =>
          CommandCopy.this.copy(
            Seq.apply[Seq[Any]](params).apply(0).apply(0).asInstanceOf[String],
            Seq.apply[Seq[Any]](params).apply(0).apply(1).asInstanceOf[String]
          )
        )),

...so my bet is that this shouldn't be CommandCopy.this.copy(...) but b.copy(...)