CapSet Type-Variable Bounds Crash the Capture Checker

[bracevac]

Compiler version

Latest nightly

Minimized code

With the recents improvements involving capture variables, we now have two ways to write down a capture variable's bounds, e.g.,

def mixedBounds[A^, B >: CapSet <: A, C >: CapSet <: CapSet^{B^}] = ???

C's bound can also be written as simply <: B. However, with the bound
<: CapSet^{B^} the setup phase of the capture checker crashes while
executing the integrateRT method.

It would be nice to support the CapSet^{B^} notation, since one expects
to be able to provide a non-singleton bound, e.g., C <: CapSet^{a,b,c}.

Output (click arrow to expand)

Exception in thread "main" java.lang.AssertionError: assertion failed: not a trackable captureRef ref: B, [B, <notype>]

  unhandled exception while running cc on tests/neg-custom-args/captures/capture-vars-subtyping.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: local/capture-bounds.scala
        during phase: cc
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.16
    compiler version: version 3.7.0-RC1-bin-SNAPSHOT-nonbootstrapped-git-c3fcd7c
            settings: -classpath /Users/oliver/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.16/scala-library-2.13.16.jar:/Users/oliver/projects/scala3/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.7.0-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.7.0-RC1-bin-SNAPSHOT.jar -d /Users/oliver/projects/scala3/compiler/../out/default-last-scalac-out.jar

        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.core.Types$BiTypeMap.ensureTrackable$1(Types.scala:6085)
        at dotty.tools.dotc.core.Types$BiTypeMap.forward(Types.scala:6086)
        at dotty.tools.dotc.core.Types$BiTypeMap.forward$(Types.scala:6069)
        at dotty.tools.dotc.cc.Setup$SubstParams.forward(Setup.scala:384)
        at dotty.tools.dotc.cc.CaptureSet.$anonfun$6(CaptureSet.scala:293)
        at dotty.tools.dotc.util.SimpleIdentitySet$Set1.map(SimpleIdentitySet.scala:86)
        at dotty.tools.dotc.cc.CaptureSet.map(CaptureSet.scala:293)
        at dotty.tools.dotc.core.Types$TypeMap.mapCapturingType(Types.scala:6168)
        at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:6221)
        at dotty.tools.dotc.cc.Setup$SubstParams.apply(Setup.scala:401)
        at dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:6212)
        at dotty.tools.dotc.cc.Setup$SubstParams.apply(Setup.scala:401)
        at dotty.tools.dotc.cc.Setup$$anon$4.integrateRT$1$$anonfun$1$$anonfun$1(Setup.scala:583)
        at scala.collection.immutable.List.map(List.scala:251)
        at dotty.tools.dotc.cc.Setup$$anon$4.integrateRT$1$$anonfun$1(Setup.scala:583)
        at dotty.tools.dotc.core.Types$PolyType.<init>(Types.scala:4346)
        at dotty.tools.dotc.core.Types$PolyType$.apply(Types.scala:4456)
        at dotty.tools.dotc.core.Types$PolyType$.apply(Types.scala:4453)
        at dotty.tools.dotc.cc.Setup$$anon$4.integrateRT$1(Setup.scala:586)
        at dotty.tools.dotc.cc.Setup$$anon$4.postProcess(Setup.scala:598)
        at dotty.tools.dotc.cc.Setup$$anon$4.traverse(Setup.scala:513)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1339)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1760)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1802)
        at dotty.tools.dotc.cc.Setup$$anon$4.traverse(Setup.scala:512)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1801)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1801)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1757)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1802)
        at dotty.tools.dotc.cc.Setup$$anon$4.traverse(Setup.scala:502)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1339)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1340)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1337)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1800)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1337)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1342)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1334)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1766)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1802)
        at dotty.tools.dotc.cc.Setup$$anon$4.traverse(Setup.scala:512)
        at dotty.tools.dotc.cc.Setup.setupUnit(Setup.scala:837)
        at dotty.tools.dotc.cc.CheckCaptures$CaptureChecker.checkUnit(CheckCaptures.scala:1548)
        at dotty.tools.dotc.transform.Recheck.run(Recheck.scala:149)
        at dotty.tools.dotc.cc.CheckCaptures.run(CheckCaptures.scala:258)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:383)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:334)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:376)
        at dotty.tools.dotc.transform.Recheck.runOn(Recheck.scala:153)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1324)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:383)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:395)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:395)
        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.dotc.Main.main(Main.scala)

[Linyxus]

I had a look. The problem seems to be with ensureTrackable in BiTypeMap at Types.scala: in integrateRT when mapping parameter references we use a BiTypeMap, which, when applied, calls ensureTrackable to assert that the results are trackable capture references. However, in integrateRT at this callsite, the info of TypeParamRef is not reliable (it seems to be NoType) as the lambda is still being formed. So this assertion always fails.

This assertion seems a sanity check, and not essential for checking the wellformedness of capture sets. Given that the info of TypeParamRef at that moment is not informative enough for us to properly perform this check, maybe we should fix this issue by temporarily disabling this assert?

[noti0na1]

The current integrateRT is not able to handle depedent parameters (both type and term) in the same list, because the mapped ref is refering to a method type which is still under constructing.

[bracevac]

It does work when we remove the isTrackableRef checks in BiTypeMap #22451. The change does not break any tests. Can we find an example that shows we need the retain the check?

Edit: I left the check in place, and instead skip it only for type and term refs that have no info.