Separation Checker: Crash when prefixing a read-only capture-variable through a self-type annotation

[bracevac]

Compiler version

Current nightly at b078f6d

Minimized code

import language.experimental.captureChecking
import caps.*

trait Zone extends Capability:
  self =>
    type R^
    def alloc: Any^{self.R.rd}

Output (click arrow to expand)

Exception in thread "main" scala.MatchError: TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),trait Zone)),type R) (of class dotty.tools.dotc.core.Types$CachedTypeRef)

  unhandled exception while running cc on tests/pending/regions.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: tests/pending/regions.scala
        during phase: cc
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.16
    compiler version: version 3.7.2-RC1-bin-SNAPSHOT-nonbootstrapped-git-b078f6d
            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/dotty/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.7.2-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.7.2-RC1-bin-SNAPSHOT.jar -d /Users/oliver/projects/dotty/compiler/../out/default-last-scalac-out.jar
        at dotty.tools.dotc.cc.Capabilities$Capability.readOnly(Capability.scala:258)
        at dotty.tools.dotc.cc.Capabilities$Capability.readOnly$(Capability.scala:235)
        at dotty.tools.dotc.core.Types$TypeRef.readOnly(Types.scala:2943)
        at dotty.tools.dotc.cc.CaptureOps$package$.toCapabilities$$anonfun$2(CaptureOps.scala:61)
        at scala.collection.immutable.List.map(List.scala:247)
        at dotty.tools.dotc.cc.CaptureOps$package$.toCapabilities(CaptureOps.scala:61)
        at dotty.tools.dotc.cc.CaptureOps$package$.$anonfun$1(CaptureOps.scala:82)
        at scala.collection.immutable.List.flatMap(List.scala:294)
        at dotty.tools.dotc.cc.CaptureOps$package$.toCaptureSet(CaptureOps.scala:82)
        at dotty.tools.dotc.cc.Setup$toCapturing$2$.innerApply(Setup.scala:449)
        at dotty.tools.dotc.cc.Setup$SetupTypeMap.apply(Setup.scala:210)
        at dotty.tools.dotc.cc.Setup$SetupTypeMap.apply$(Setup.scala:196)
        at dotty.tools.dotc.cc.Setup$toCapturing$2$.apply(Setup.scala:378)
        at dotty.tools.dotc.cc.Setup.transform$1(Setup.scala:476)
        at dotty.tools.dotc.cc.Setup.dotty$tools$dotc$cc$Setup$$transformExplicitType(Setup.scala:482)
        at dotty.tools.dotc.cc.Setup$$anon$3.transformTT(Setup.scala:537)
        at dotty.tools.dotc.cc.Setup$$anon$3.transformResultType(Setup.scala:547)
        at dotty.tools.dotc.cc.Setup$$anon$3.traverse(Setup.scala:576)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1346)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1806)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1347)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1806)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1347)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1766)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1808)
        at dotty.tools.dotc.cc.Setup$$anon$3.traverse(Setup.scala:622)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1807)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.apply(Trees.scala:1807)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1763)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1808)
        at dotty.tools.dotc.cc.Setup$$anon$3.traverse(Setup.scala:601)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1346)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1806)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1344)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverse(Trees.scala:1806)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.recur$2(tpd.scala:1344)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1349)
        at dotty.tools.dotc.ast.tpd$TreeTraverserWithPreciseImportContexts.apply(tpd.scala:1341)
        at dotty.tools.dotc.ast.Trees$Instance$TreeAccumulator.foldOver(Trees.scala:1772)
        at dotty.tools.dotc.ast.Trees$Instance$TreeTraverser.traverseChildren(Trees.scala:1808)
        at dotty.tools.dotc.cc.Setup$$anon$3.traverse(Setup.scala:622)
        at dotty.tools.dotc.cc.Setup.setupUnit(Setup.scala:927)
        at dotty.tools.dotc.cc.CheckCaptures$CaptureChecker.checkUnit(CheckCaptures.scala:1759)
        at dotty.tools.dotc.transform.Recheck.run(Recheck.scala:147)
        at dotty.tools.dotc.cc.CheckCaptures.run(CheckCaptures.scala:231)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:383)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:334)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:376)
        at dotty.tools.dotc.transform.Recheck.runOn(Recheck.scala:151)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:368)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1324)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:361)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1$$anonfun$2(Run.scala:408)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1$$anonfun$adapted$1(Run.scala:408)
        at scala.Function0.apply$mcV$sp(Function0.scala:42)
        at dotty.tools.dotc.Run.showProgress(Run.scala:470)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:408)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:420)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:420)
        at dotty.tools.dotc.Run.compileSources(Run.scala:307)
        at dotty.tools.dotc.Run.compile(Run.scala:292)
        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)

[bracevac]

The same happens with this.R.rd. At least, it should behave as if we did not provide the prefix

-- Error: tests/pending/regions.scala:79:20 ------------------------------------
79 |    def alloc: Any^{R.rd}
   |                    ^^^^
   |_root_.scala.caps.internal.capsOf[Zone.this.R] cannot be tracked since it is not a parameter or local value
1 error found

But more generally, it's not so clear why we get the error here. Should this not be allowed?

[bracevac]

There seem to be more issues regarding this and self-type annotations:

trait Zone extends caps.Capability:
    type R^
    def alloc: Any^{this}

Will compile just fine, but

trait Zone extends caps.Capability:
  self =>
    type R^
    def alloc: Any^{self}

results in a warning:

-- Warning: tests/pending/regions.scala:79:20 ----------------------------------
79 |    def alloc: Any^{self}
   |                    ^^^^
   |          redundant capture: Any already accounts for (Zone.this : Zone)
1 warning found