Simple snippet crashing when Ycheck:all

[szymon-rd]

Compiler version

3.4.0-RC1-bin-20231109-c7b3d7b-NIGHTLY

Minimized code

//> using options -Ycheck:all

trait B(val y: Int)

class C extends B(20)  {
  def foo(): Unit = println(y)
}

This code comes from tests/init/neg/trait1. More snippets like this are crashing. After implementing #18634 the pipeline goes on instead of failing on the first warn, and the compiler crashes.

Output (click arrow to expand)

Fatal compiler crash when compiling: tests/init/neg/trait1.scala:
assertion failed: bad type (C.this.y : (): Int) for C.this.y # -1
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedIdent(TreeChecker.scala:435)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3100)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3215)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:174)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:398)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3293)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3297)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:381)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3409)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:958)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1118)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:352)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedApply(TreeChecker.scala:515)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3132)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3216)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:174)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:398)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.$anonfun$7(ProtoTypes.scala:509)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.cacheTypedArg(ProtoTypes.scala:432)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:510)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:914)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:914)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:606)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:670)

[szymon-rd]

y Ident in line with def foo in class C stops being ParamAccessor after megaphase with elimErasedValueType for some reason, and needsSelect starts returning true (because due to the lack of ParamAccessor flag prefixIsElidable becomes false for the type of Ident y)

Edit: It changes in mixin phase, it drops the ParamAccessor

[szymon-rd]

If I understand it correctly, prefixIsElidable assumes that the ParamAccessor flag is not dropped. But then, the logic in Mixin changes that. Is there any other way to know if the prefix is elidable after Mixin (in an efficient way)? @odersky