Compiler crashes when value class extends an alias of `AnyVal`

[hamzaremmal]

Compiler version

cc38962

Minimized code

type AliasToAnyVal = AnyVal
class Foo(a: Int) extends AliasToAnyVal

Output (click arrow to expand)

scalac anyval.scala

  unhandled exception while running erasure on anyval.scala

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/scala/scala3/issues/new/choose
  For non-enriched exceptions, compile with -Xno-enrich-error-messages.

     while compiling: anyval.scala
        during phase: erasure
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.14
    compiler version: version 3.5.0
            settings:

Exception in thread "main" java.lang.AssertionError: NoDenotation.owner
	at dotty.tools.dotc.core.SymDenotations$NoDenotation$.owner(SymDenotations.scala:2623)
	at dotty.tools.dotc.ast.tpd$.ref(tpd.scala:462)
	at dotty.tools.dotc.transform.Erasure$Boxing$.wrap$1(Erasure.scala:338)
	at dotty.tools.dotc.transform.Erasure$Boxing$.cast(Erasure.scala:366)
	at dotty.tools.dotc.transform.Erasure$Boxing$.unbox(Erasure.scala:320)
	at dotty.tools.dotc.transform.Erasure$Boxing$.adaptToType(Erasure.scala:403)
	at dotty.tools.dotc.transform.Erasure$Typer.adapt(Erasure.scala:1098)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3669)
	at dotty.tools.dotc.transform.Erasure$Typer.$anonfun$7(Erasure.scala:863)
	at dotty.tools.dotc.core.Decorators$.zipWithConserve(Decorators.scala:160)
	at dotty.tools.dotc.transform.Erasure$Typer.typedApply(Erasure.scala:863)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3392)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3477)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3669)
	at dotty.tools.dotc.transform.Erasure$Typer.typedApply(Erasure.scala:850)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3392)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3477)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3669)
	at dotty.tools.dotc.typer.Typer.$anonfun$64(Typer.scala:2816)
	at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:256)
	at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2816)
	at dotty.tools.dotc.transform.Erasure$Typer.typedDefDef(Erasure.scala:967)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3374)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3476)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3580)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3626)
	at dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1080)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:3074)
	at dotty.tools.dotc.transform.Erasure$Typer.typedClassDef(Erasure.scala:1056)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3380)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3384)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3476)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3580)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3626)
	at dotty.tools.dotc.transform.Erasure$Typer.typedStats(Erasure.scala:1080)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:3207)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3426)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3477)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3554)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3558)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3669)
	at dotty.tools.dotc.transform.Erasure.run(Erasure.scala:145)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:380)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:373)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:384)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:396)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:396)
	at dotty.tools.dotc.Run.compileSources(Run.scala:282)
	at dotty.tools.dotc.Run.compile(Run.scala:267)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:201)
	at dotty.tools.dotc.Driver.process(Driver.scala:169)
	at dotty.tools.dotc.Driver.process(Driver.scala:181)
	at dotty.tools.dotc.Driver.main(Driver.scala:211)
	at dotty.tools.MainGenericCompiler$.run$1(MainGenericCompiler.scala:162)
	at dotty.tools.MainGenericCompiler$.main(MainGenericCompiler.scala:186)
	at dotty.tools.MainGenericCompiler.main(MainGenericCompiler.scala)

Notes

The check to determine if a class is a value class doesn't take aliasing into account. See:

scala3/compiler/src/dotty/tools/dotc/ast/Desugar.scala

Lines 597 to 601 in cc38962

	def isAnyVal(tree: Tree): Boolean = tree match {
	case Ident(tpnme.AnyVal) => true
	case Select(qual, tpnme.AnyVal) => isScala(qual)
	case _ => false
	}

[odersky]

The extends AnyVal is a crutch anyway, we should have explicit syntax for that. So I would just outlaw this case and forbid inheriting aliases of AnyVal.