Compiler crash when trying to use object with self reference in multi stage compilation

[WojciechMazur]

Self types in objects are prohibited in Scala 3 so probably this is an unhandled corner case. Adding type annotation results in compilation error

Compiler version

3.2.1
3.3.0-RC1-bin-20221128-8fba321-NIGHTLY

Minimized code

// definition_1.scala
class OU {
  def dn: String = ""
}

class Inventory() {
  object NODES extends OU { nodes =>
    val x = nodes.dn
  }
}

// usage_2.scala
@main def Test =
  new Inventory().NODES.dn

Output (click arrow to expand)

exception while typing new Inventory().NODES.dn of class class dotty.tools.dotc.ast.Trees$Select # -1
exception while typing @main def Test: String = new Inventory().NODES.dn of class class dotty.tools.dotc.ast.Trees$DefDef # -1
exception while typing @SourceFile("tests/run/custom-sandbox/usage_2.scala") final module class usage_2$package() extends Object() {
  private def writeReplace(): AnyRef = new scala.runtime.ModuleSerializationProxy(classOf[usage_2$package.type])
  @main def Test: String = new Inventory().NODES.dn
} of class class dotty.tools.dotc.ast.Trees$TypeDef # -1
exception while typing package <empty> {
  final lazy module val usage_2$package: usage_2$package = new usage_2$package()
  @SourceFile("tests/run/custom-sandbox/usage_2.scala") final module class usage_2$package() extends Object() {
    private def writeReplace(): AnyRef = new scala.runtime.ModuleSerializationProxy(classOf[usage_2$package.type])
    @main def Test: String = new Inventory().NODES.dn
  }
  @SourceFile("tests/run/custom-sandbox/usage_2.scala") final class Test() extends Object() {
    <static> def main(args: Array[String]): Unit =
      try
        {
          Test
          ()
        }
       catch
        {
          case error @ _:scala.util.CommandLineParser.ParseError => scala.util.CommandLineParser.showError(error)
        }
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # -1
scala.MatchError: ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),class Inventory)),module class NODES$)) (of class dotty.tools.dotc.core.Types$CachedThisType) while running erasure on tests/run/custom-sandbox/usage_2.scala
scala.MatchError: ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),class Inventory)),module class NODES$)) (of class dotty.tools.dotc.core.Types$CachedThisType) while compiling tests/run/custom-sandbox/usage_2.scala
Fatal compiler crash when compiling: tests/run/custom-sandbox:
ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),class Inventory)),module class NODES$)) (of class dotty.tools.dotc.core.Types$CachedThisType)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.sourceOfSelf$1(SymDenotations.scala:1055)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.sourceModule(SymDenotations.scala:1057)
        at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:680)
        at dotty.tools.dotc.core.TypeErasure.eraseInfo(TypeErasure.scala:745)
        at dotty.tools.dotc.core.TypeErasure$.transformInfo(TypeErasure.scala:244)
        at dotty.tools.dotc.transform.Erasure.transform(Erasure.scala:97)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:833)
        at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:882)
        at dotty.tools.dotc.core.Symbols$Symbol.recomputeDenot(Symbols.scala:120)
        at dotty.tools.dotc.core.Symbols$Symbol.computeDenot(Symbols.scala:114)
        at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:107)
        at dotty.tools.dotc.core.Symbols$.toDenot(Symbols.scala:494)
        at dotty.tools.dotc.transform.TypeUtils$.isErasedClass(TypeUtils.scala:27)
        at dotty.tools.dotc.transform.Erasure$Typer.checkNotErasedClass(Erasure.scala:579)
        at dotty.tools.dotc.transform.Erasure$Typer.checkNotErasedClass(Erasure.scala:587)
        at dotty.tools.dotc.transform.Erasure$Typer.checkNotErased(Erasure.scala:572)
        at dotty.tools.dotc.transform.Erasure$Typer.typedSelect(Erasure.scala:789)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2869)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2962)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3030)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3034)
        at dotty.tools.dotc.transform.Erasure$Typer.typedSelect(Erasure.scala:667)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2869)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2962)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3030)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3034)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3150)
        at dotty.tools.dotc.typer.Typer.$anonfun$51(Typer.scala:2358)
        at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:249)
        at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2358)
        at dotty.tools.dotc.transform.Erasure$Typer.typedDefDef(Erasure.scala:944)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2876)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2962)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3030)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3034)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3056)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3106)
        at dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1043)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2534)
        at dotty.tools.dotc.transform.Erasure$Typer.typedClassDef(Erasure.scala:1032)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:2888)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2892)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2962)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3030)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3034)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3056)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3106)
        at dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1043)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2664)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2933)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2963)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:126)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3030)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3034)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3150)
        at dotty.tools.dotc.transform.Erasure.run(Erasure.scala:144)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:324)
        at scala.collection.immutable.List.map(List.scala:246)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:328)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:244)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:260)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:268)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:277)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:277)
        at dotty.tools.dotc.Run.compileSources(Run.scala:195)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
        at dotty.tools.dotc.Driver.process(Driver.scala:195)
        at dotty.tools.dotc.Driver.process(Driver.scala:163)
        at dotty.tools.vulpix.ParallelTesting$Test.compile(ParallelTesting.scala:533)
        at dotty.tools.vulpix.ParallelTesting$CompilationLogic.compileTestSource$$anonfun$1$$anonfun$1(ParallelTesting.scala:236)
        at scala.collection.immutable.List.map(List.scala:250)
        at dotty.tools.vulpix.ParallelTesting$CompilationLogic.compileTestSource$$anonfun$1(ParallelTesting.scala:238)
        at scala.util.Try$.apply(Try.scala:210)
        at dotty.tools.vulpix.ParallelTesting$CompilationLogic.dotty$tools$vulpix$ParallelTesting$CompilationLogic$$compileTestSource(ParallelTesting.scala:239)
        at dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource$$anonfun$1(ParallelTesting.scala:285)
        at dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource$$anonfun$adapted$1(ParallelTesting.scala:288)
        at scala.Function0.apply$mcV$sp(Function0.scala:42)
        at dotty.tools.vulpix.ParallelTesting$Test.tryCompile(ParallelTesting.scala:462)
        at dotty.tools.vulpix.ParallelTesting$$anon$4.checkTestSource(ParallelTesting.scala:288)
        at dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run(ParallelTesting.scala:358)
        at dotty.tools.vulpix.ParallelTesting$Test$LoggedRunnable.run$(ParallelTesting.scala:340)
        at dotty.tools.vulpix.ParallelTesting$$anon$4.run(ParallelTesting.scala:283)
        at java.base/java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1407)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:183)```

Adding type annotation would result in compilation error:

-- [E013] Syntax Error: /home/wmazur/projects/virtuslab/normation/rudder/webapp/sources/test.sc:15:28 --------------------------------------------------------------------------------------------------------------------------------------------------
15 |  object NODES extends OU { servers: OU =>
   |                            ^^^^^^^^^^^^^^
   |                            objects must not have a self type

[Kordyjan]

This require fixing the condition at
https://github.com/lampepfl/dotty/blob/7b9c1a85f1ca227e7a0b9971bf3b74f575a2bf8b/compiler/src/dotty/tools/dotc/ast/Desugar.scala#L901

[dwijnand]

I'd say constraining an object's self type is prohibited. Every object, every class instance in general, has a self type, called this be default. So I wouldn't make that code not compile. I'd look to handling this in the match that blows up (in sourceOfSelf). case tp: ThisType => tp.cls I think.