Compiler crash during explicitOuter

[liufengyun]

The following code crashes Dotty:

class A(a: Int) {
  class X {
    val x = a
  }

  trait Y {
    val y = a
  }

  trait Z(val o: A) extends o.Y {
    val z = a
  }

  class B extends X with Z(new A(4))
}

error message

Exception in thread "main" java.lang.AssertionError: assertion failed: failure to construct path from method A$Y$$$outer/class B/class A/package /package  to `this` of trait Z in class A;
class A does not have an outer accessor
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:36)
	at dotty.tools.dotc.transform.ExplicitOuter$OuterOps$.loop$1(ExplicitOuter.scala:388)
	at dotty.tools.dotc.transform.ExplicitOuter$OuterOps$.path$extension(ExplicitOuter.scala:394)
	at dotty.tools.dotc.transform.LambdaLift$Lifter.memberRef(LambdaLift.scala:400)
	at dotty.tools.dotc.transform.LambdaLift.transformIdent(LambdaLift.scala:519)
	at dotty.tools.dotc.transform.MegaPhase.goIdent(MegaPhase.scala:518)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:210)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:366)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:250)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:367)
	at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:230)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:233)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:366)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:375)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:380)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:380)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:319)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:367)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:236)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:366)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:375)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:380)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:380)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:319)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:367)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:236)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:366)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:375)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:380)
	at scala.collection.immutable.List.mapConserve(List.scala:176)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:380)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:336)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:339)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:367)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:386)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:398)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:295)
	at scala.collection.immutable.List.map(List.scala:283)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:297)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:171)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:18)
	at scala.compat.java8.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:191)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:186)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:194)
	at scala.compat.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:88)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:201)
	at dotty.tools.dotc.Run.compileSources(Run.scala:136)
	at dotty.tools.dotc.Run.compile(Run.scala:120)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:29)
	at dotty.tools.dotc.Driver.process(Driver.scala:127)
	at dotty.tools.dotc.Driver.process(Driver.scala:96)
	at dotty.tools.dotc.Driver.process(Driver.scala:108)
	at dotty.tools.dotc.Driver.main(Driver.scala:135)
	at dotty.tools.dotc.Main.main(Main.scala)

[liufengyun]

As discussed in #5099, we should disallow usage of trait parameters as outers for parent classes.