"Double definition: final def ...$$outer" when compiling with Scala.js (but not with the JVM backend)

[smarter]

This is not minimized but can be reproduced in the scalacheck repo, I'm hoping @sjrd can figure out what this is anyway 😅 :

git clone https://github.com/smarter/scalacheck/ -b 3.0.0-M1-and-scalajs
cd scalacheck
sbt "++3.0.0-M1 js/compile"

Output

[...]
[error] -- [E120] Naming Error: /home/smarter/opt/scalacheck/src/main/scala/org/scalacheck/commands/Commands.scala:98:8
[error] 98 |  trait Command {
[error]    |        ^
[error]    |Double definition:
[error]    |final def org$scalacheck$commands$Commands$Command$$$outer
[error]    |  (): org.scalacheck.commands.Commands in class CommandSequence at line 98 and
[error]    |final def org$scalacheck$commands$Commands$Command$$$outer
[error]    |  (): org.scalacheck.commands.Commands in class CommandSequence at line 98
[error]    |have the same type after erasure.
[error] -- [E120] Naming Error: /home/smarter/opt/scalacheck/src/main/scala/org/scalacheck/commands/Commands.scala:137:8
[error] 137 |  trait SuccessCommand extends Command {
[error]     |        ^
[error]     |Double definition:
[error]     |final def org$scalacheck$commands$Commands$SuccessCommand$$$outer
[error]     |  (): org.scalacheck.commands.Commands in class CommandSequence at line 137 and
[error]     |final def org$scalacheck$commands$Commands$SuccessCommand$$$outer
[error]     |  (): org.scalacheck.commands.Commands in class CommandSequence at line 137
[error]     |have the same type after erasure.

[sjrd]

Ouch, that does not look good :(

Workaround, since ScalaCheck does not have any non-native JS type anyway: add the following to the settings of the js project:

scalacOptions ++= if (scalaVersion.value == "3.0.0-M1") Seq("-Yskip:explicitJSClasses") else Nil,

[sjrd]

Here is a minimization:

class OuterClass { // can also be trait
  trait ParentTrait { // must be trait, can be private
    def concreteMethod(x: Int): Int = x // must have a concrete method
  }

  private class ChildClass extends ParentTrait // must be class *and* private
}

It requires a private class extending a trait with a concrete method, both in an enclosing class or trait.