Opaque type failing Ycheck when compiling from TASTy

[nicolasstucki]

minimized code

trait CompilerInterface {
  type Tree
}

class Reflect(val internal: CompilerInterface) {
  opaque type Tree = internal.Tree
  def show(t: Tree): String = ???
}

object App {
  val refl: Reflect = ???
  import refl._

  show(??? : Tree)
}

This code fails to Ycheck when compiling from TASTy.

It can be reproduced using the following command from sbt

dotc -Ythrough-tasty -Ycheck:all XYZ.scala

checking /Users/nicolasstucki/GitHub/dotty/out/from-tasty-tmp7774217330379414850/from-source/App.class after phase MegaPhase{elimOpaque, tryCatchPatterns, patternMatcher, explicitOuter, explicitSelf, stringInterpolatorOpt, crossCast}
exception while typing App.refl.show(??? :App.refl.Tree) of class class dotty.tools.dotc.ast.Trees$Apply # 2116
exception while typing @scala.annotation.internal.SourceFile(
  "tests/pos/tasty-reflect-opaque-api-proto.scala"
) final module class App$() extends Object(), Serializable {
  private def writeReplace(): AnyRef = 
    new runtime.ModuleSerializationProxy(classOf[App.type])
  val refl: Reflect = ???
  App.refl.show(??? :App.refl.Tree)
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 2232
exception while typing package <empty> {
  final lazy module val App: App$ = new App$()
  @scala.annotation.internal.SourceFile(
    "tests/pos/tasty-reflect-opaque-api-proto.scala"
  ) final module class App$() extends Object(), Serializable {
    private def writeReplace(): AnyRef = 
      new runtime.ModuleSerializationProxy(classOf[App.type])
    val refl: Reflect = ???
    App.refl.show(??? :App.refl.Tree)
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 2233
*** error while checking /Users/nicolasstucki/GitHub/dotty/out/from-tasty-tmp7774217330379414850/from-source/App.class after phase crossCast ***
java.lang.AssertionError: assertion failed: Found:    App.refl.Tree
Required: App.refl.Tree
tree = ??? :App.refl.Tree whileException in thread "main"  compiling Reflect, App, CompilerInterfacejava.lang.AssertionError: assertion failed: Found:    App.refl.Tree
Required: App.refl.Tree
tree = ??? :App.refl.Tree
        at dotty.DottyPredef$.assertFail(DottyPredef.scala:16)
        at dotty.tools.dotc.transform.TreeChecker$Checker.adapt(TreeChecker.scala:508)
        at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:328)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:795)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:795)
        at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:532)
        at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:601)
        at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:353)
        at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:692)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:794)
        at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:867)
        at dotty.tools.dotc.typer.Applications.realApply$4$$anonfun$3(Applications.scala:896)
        at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:2251)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:907)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:942)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:83)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2063)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2119)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:123)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:302)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2155)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2167)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:287)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2208)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2230)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:476)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1730)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:416)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2053)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2118)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:123)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:302)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2155)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2167)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:287)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2186)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2230)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:476)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1854)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2094)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2119)
        at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:123)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:302)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2155)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2153)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2167)
        at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:287)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2243)
        at dotty.tools.dotc.transform.TreeChecker.check(TreeChecker.scala:122)
        at dotty.tools.dotc.transform.TreeChecker.run(TreeChecker.scala:94)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:315)
        at scala.collection.immutable.List.map(List.scala:226)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:316)
        at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:159)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1350)
        at dotty.tools.dotc.Run.runPhases$5(Run.scala:169)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:177)
        at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:184)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:126)
        at dotty.tools.dotc.fromtasty.TASTYRun.compile(TASTYRun.scala:10)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
        at dotty.tools.dotc.Driver.process(Driver.scala:179)
        at dotty.tools.dotc.Driver.process(Driver.scala:148)
        at dotty.tools.dotc.Driver.process(Driver.scala:160)
        at dotty.tools.dotc.fromtasty.Debug$.main(Debug.scala:53)
        at dotty.tools.dotc.fromtasty.Debug.main(Debug.scala)

expectation

Ycheck should pass.

[nicolasstucki]

I noticed that in TreeChecker$Checker.adapt(TreeChecker.scala:508) where the error is emitted the prototype pt of type App.refl.Tree can be dealiased to App.refl.internal.Tree which seem to indicate the the type is not opaque at this point.

[nicolasstucki]

This was discovered in #7158

[odersky]

I noticed that in TreeChecker$Checker.adapt(TreeChecker.scala:508) where the error is emitted the prototype pt of type App.refl.Tree can be dealiased to App.refl.internal.Tree which seem to indicate the the type is not opaque at this point.

That's correct and expected. The phase where this happens is after ElimOpaque.

[nicolasstucki]

Then why the type of the tree does not dealias to App.refl.internal.Tree as well? Possibly ElimOpaque missed it which might be related to ElimOpaque.scala#L50-L53

[odersky]

@nicolasstucki Yes, it was indeed related to https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/transform/ElimOpaque.scala#L50-L53
But before we could even trigger the correct behavior we had to fix the way prefixes are handled in Denotations.