Assert when compiling a complex combination of traits and objects

[OndrejSpanel]

Reproduction steps

I do not have a small repro yet, the issue is reproducible in my complex project.

Problem

When compiling the project, which is using Scala 2.13.8, I get following error:

java.lang.AssertionError: assertion failed: 
  List(object package$EntityInScene, object package$EntityInScene)
     while compiling: W:\Tempi\threeJVM\src\test\scala\net\gamatron\procas\game\state\AbstractStateTest.scala
        during phase: globalPhase=mixin, enteringPhase=cleanup

The call-stack for the assertion seems to be:

scala.reflect.internal.SymbolTable.throwAssertionError(SymbolTable.scala:171)
scala.reflect.internal.Symbols$Symbol.suchThat(Symbols.scala:2040)
scala.reflect.internal.Symbols$ClassSymbol.companionModule0(Symbols.scala:3391)
scala.reflect.internal.Symbols$ClassSymbol.companionSymbol(Symbols.scala:3394)
scala.tools.nsc.typechecker.Namers.companionSymbolOf(Namers.scala:2180)
scala.tools.nsc.typechecker.Namers.companionSymbolOf$(Namers.scala:2171)
scala.tools.nsc.transform.Erasure.companionSymbolOf(Erasure.scala:23)
scala.tools.nsc.typechecker.Namers.linkedClassOfClassOf(Namers.scala:2190)
scala.tools.nsc.typechecker.Namers.linkedClassOfClassOf$(Namers.scala:2186)
scala.tools.nsc.transform.Erasure.linkedClassOfClassOf(Erasure.scala:23)
scala.tools.nsc.typechecker.Contexts$Context.accessWithinLinked$1(Contexts.scala:910)
scala.tools.nsc.typechecker.Contexts$Context.isAccessible(Contexts.scala:965)
scala.tools.nsc.typechecker.Infer$Inferencer.$anonfun$checkAccessible$1(Infer.scala:289)
scala.tools.nsc.typechecker.Infer$Inferencer.$anonfun$checkAccessible$1$adapted(Infer.scala:289)
scala.reflect.internal.Symbols$Symbol.filter(Symbols.scala:2035)
scala.tools.nsc.typechecker.Infer$Inferencer.accessible$1(Infer.scala:289)
scala.tools.nsc.typechecker.Infer$Inferencer.checkAccessible(Infer.scala:298)
scala.tools.nsc.typechecker.Typers$Typer.makeAccessible(Typers.scala:577)
scala.tools.nsc.typechecker.Typers$Typer.$anonfun$typed1$62(Typers.scala:5343)
scala.tools.nsc.typechecker.Typers$Typer.silent(Typers.scala:712)
scala.tools.nsc.typechecker.Typers$Typer.typedSelect$1(Typers.scala:5343)
scala.tools.nsc.typechecker.Typers$Typer.typedSelectOrSuperCall$1(Typers.scala:5411)
scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:5986)
scala.tools.nsc.transform.Erasure$Eraser.typed1(Erasure.scala:806)
scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:6041)
scala.tools.nsc.typechecker.Typers$Typer.$anonfun$typed1$41(Typers.scala:5074)
scala.tools.nsc.typechecker.Typers$Typer.silent(Typers.scala:698)
scala.tools.nsc.typechecker.Typers$Typer.normalTypedApply$1(Typers.scala:5076)
scala.tools.nsc.typechecker.Typers$Typer.typedApply$1(Typers.scala:5104)
scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:5985)
scala.tools.nsc.transform.Erasure$Eraser.typed1(Erasure.scala:806)
scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:6041)
scala.tools.nsc.typechecker.Typers$Typer.typedDefDef(Typers.scala:6318)
scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:5950)
scala.tools.nsc.transform.Erasure$Eraser.typed1(Erasure.scala:806)

...

For the error to happen I need to use a source similar to (but much more complex):

package object foo {

  trait Foo {
    def f(x: Double): String
  }

  object Foo {
    trait FooA extends Foo {
      def f(x: Double): String = x.toString
    }
    trait FooB extends Foo {
      def f(x: Double): String = x.toInt.toString
    }
  }

  case class BarB() extends Foo with Foo.FooB
  case class BarA() extends Foo with Foo.FooA
}

Once I move FooA and FooB out of the the Foo companion, everything works fine.

Does this ring the bell for anyone? Is it worth spending an effort creating a proper repro?

[som-snytt]

There's a lint about not putting classes in package objects. I don't know if there is still an open ticket about the perils to which one is exposed.

[OndrejSpanel]

I confirm when I move the object Foo out of the package object foo, and make a separate source file in the package foo for it, I get no assertion and the project compiles and runs fine.

[som-snytt]

The old ticket with same stack trace. #5954 (asserting that the companion is not overloaded)

The open umbrella ticket wherein it is hoped that the other ticket was resolved. #4344