TyperState crash: search for extension method in implicit scope

[m8nmueller]

Compiler version

3.3.1

Minimized code

trait SuspendSupport:
  type Suspension[-T, +R]
  extension [T, R](s: Suspension[T, R])
    def resume(arg: T): R

  def suspend[T, R](body: Suspension[T, R] => R): T

trait Async(using val support: SuspendSupport)

private class YZ(using ac: Async):
  private def test =
    Some(()).getOrElse:
      ac.support.suspend[scala.util.Try[Unit], Unit](k =>
        k.resume(scala.util.Failure(Exception()))
        1
      )

Output (click arrow to expand)

java.lang.AssertionError: assertion failed: TS[1, 0] attempted to take ownership of T which is already owned by committable TS[12, 1, 0] while typechecking test.scala
exception occurred while typechecking test.scala

  exception occurred while compiling List(test.scala)

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/lampepfl/dotty/issues/new/choose

     while compiling: <no file>
        during phase: <no phase>
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.10
    compiler version: version 3.3.1
            settings: 

                tree: EmptyTree
       tree position: :<unknown>
           tree type: <notype>
              symbol: val <none>
           call site: package <root> in module class <root>

  == Source file context for tree position ==


Exception in thread "main" java.lang.AssertionError: assertion failed: TS[1, 0] attempted to take ownership of T which is already owned by committable TS[12, 1, 0]
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.TyperState.includeVar(TyperState.scala:271)
	at dotty.tools.dotc.core.TyperState.$anonfun$2$$anonfun$1(TyperState.scala:233)
	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:333)
	at dotty.tools.dotc.core.TyperState.$anonfun$2(TyperState.scala:233)
	at scala.collection.immutable.List.forall(List.scala:386)
	at dotty.tools.dotc.core.TyperState.mergeConstraintWith(TyperState.scala:235)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArgs(ProtoTypes.scala:480)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.fold(ProtoTypes.scala:566)
	at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6250)
	at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6288)
	at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:640)
	at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6287)
	at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6287)
	at dotty.tools.dotc.typer.ProtoTypes$SelectionProto.fold(ProtoTypes.scala:244)
	at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6250)
	at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6288)
	at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:640)
	at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6287)
	at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6287)
	at dotty.tools.dotc.typer.ProtoTypes$ViewProto.fold(ProtoTypes.scala:623)
	at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6250)
	at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6288)
	at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:640)
	at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.apply(Implicits.scala:645)
	at dotty.tools.dotc.typer.ImplicitRunInfo.recur$1(Implicits.scala:721)
	at dotty.tools.dotc.typer.ImplicitRunInfo.computeIScope(Implicits.scala:732)
	at dotty.tools.dotc.typer.ImplicitRunInfo.implicitScope(Implicits.scala:804)
	at dotty.tools.dotc.typer.ImplicitRunInfo.implicitScope$(Implicits.scala:584)
	at dotty.tools.dotc.Run.implicitScope(Run.scala:36)
	at dotty.tools.dotc.typer.Implicits$ImplicitSearch.implicitScope(Implicits.scala:1575)
	at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1538)
	at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1547)
	at dotty.tools.dotc.typer.Implicits$ImplicitSearch.bestImplicit(Implicits.scala:1572)
	at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1060)
	at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:818)
	at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:116)
	at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1080)
	at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:818)
	at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:116)
	at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1080)
	at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:818)
	at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:116)
	at dotty.tools.dotc.typer.Implicits.inferView(Implicits.scala:856)
	at dotty.tools.dotc.typer.Implicits.inferView$(Implicits.scala:818)
	at dotty.tools.dotc.typer.Typer.inferView(Typer.scala:116)
	at dotty.tools.dotc.typer.Typer.tryExtensionOrConversion(Typer.scala:3527)
	at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:703)
	at dotty.tools.dotc.typer.Typer.typeSelectOnTerm$1(Typer.scala:753)
	at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:790)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3017)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3111)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3300)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:941)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1101)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:352)
	at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:116)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3048)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3237)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3256)
	at dotty.tools.dotc.typer.Typer.typedBlockStats(Typer.scala:1159)
	at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1163)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3056)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3300)
	at dotty.tools.dotc.typer.Typer.$anonfun$57(Typer.scala:2486)
	at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:243)
	at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2486)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3024)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3111)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3210)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3256)
	at dotty.tools.dotc.typer.Typer.typedBlockStats(Typer.scala:1159)
	at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1163)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3056)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.typedFunctionValue(Typer.scala:1626)
	at dotty.tools.dotc.typer.Typer.typedFunction(Typer.scala:1377)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3058)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.$anonfun$7(ProtoTypes.scala:495)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.cacheTypedArg(ProtoTypes.scala:418)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:496)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:897)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:897)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:589)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:653)
	at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:492)
	at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:779)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:896)
	at dotty.tools.dotc.typer.Applications.ApplyTo(Applications.scala:1126)
	at dotty.tools.dotc.typer.Applications.ApplyTo$(Applications.scala:352)
	at dotty.tools.dotc.typer.Typer.ApplyTo(Typer.scala:116)
	at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:969)
	at dotty.tools.dotc.typer.Applications.realApply$1$$anonfun$2(Applications.scala:1052)
	at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:3324)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1063)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1101)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:352)
	at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:116)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3048)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.$anonfun$7(ProtoTypes.scala:495)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.cacheTypedArg(ProtoTypes.scala:418)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:496)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:897)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:897)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:589)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:653)
	at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:492)
	at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:779)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:896)
	at dotty.tools.dotc.typer.Applications.ApplyTo(Applications.scala:1126)
	at dotty.tools.dotc.typer.Applications.ApplyTo$(Applications.scala:352)
	at dotty.tools.dotc.typer.Typer.ApplyTo(Typer.scala:116)
	at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:969)
	at dotty.tools.dotc.typer.Applications.realApply$1$$anonfun$2(Applications.scala:1052)
	at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:3324)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1063)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1101)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:352)
	at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:116)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3048)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3300)
	at dotty.tools.dotc.typer.Namer.typedAheadExpr$$anonfun$1(Namer.scala:1653)
	at dotty.tools.dotc.typer.Namer.typedAhead(Namer.scala:1643)
	at dotty.tools.dotc.typer.Namer.typedAheadExpr(Namer.scala:1653)
	at dotty.tools.dotc.typer.Namer.typedAheadRhs$1$$anonfun$1(Namer.scala:1906)
	at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:243)
	at dotty.tools.dotc.typer.Namer.typedAheadRhs$1(Namer.scala:1906)
	at dotty.tools.dotc.typer.Namer.rhsType$1(Namer.scala:1914)
	at dotty.tools.dotc.typer.Namer.cookedRhsType$1(Namer.scala:1932)
	at dotty.tools.dotc.typer.Namer.lhsType$1(Namer.scala:1933)
	at dotty.tools.dotc.typer.Namer.inferredResultType(Namer.scala:1944)
	at dotty.tools.dotc.typer.Namer.inferredType$1(Namer.scala:1691)
	at dotty.tools.dotc.typer.Namer.valOrDefDefSig(Namer.scala:1698)
	at dotty.tools.dotc.typer.Namer.defDefSig(Namer.scala:1789)
	at dotty.tools.dotc.typer.Namer$Completer.typeSig(Namer.scala:791)
	at dotty.tools.dotc.typer.Namer$Completer.completeInCreationContext(Namer.scala:934)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:814)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:174)
	at dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:187)
	at dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:189)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:393)
	at dotty.tools.dotc.typer.Typer.retrieveSym(Typer.scala:2989)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3014)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3111)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3210)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3256)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2669)
	at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3036)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3040)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3111)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3210)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3256)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2812)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3081)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3112)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3184)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3188)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3300)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$1(TyperPhase.scala:44)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$adapted$1(TyperPhase.scala:54)
	at scala.Function0.apply$mcV$sp(Function0.scala:42)
	at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:440)
	at dotty.tools.dotc.typer.TyperPhase.typeCheck(TyperPhase.scala:54)
	at dotty.tools.dotc.typer.TyperPhase.runOn$$anonfun$3(TyperPhase.scala:88)
	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:333)
	at dotty.tools.dotc.typer.TyperPhase.runOn(TyperPhase.scala:88)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:262)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:270)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:279)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:279)
	at dotty.tools.dotc.Run.compileSources(Run.scala:194)
	at dotty.tools.dotc.Run.compile(Run.scala:179)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:197)
	at dotty.tools.dotc.Driver.process(Driver.scala:165)
	at dotty.tools.dotc.Driver.process(Driver.scala:177)
	at dotty.tools.dotc.Driver.main(Driver.scala:207)
	at dotty.tools.MainGenericCompiler$.run$1(MainGenericCompiler.scala:162)
	at dotty.tools.MainGenericCompiler$.main(MainGenericCompiler.scala:186)
	at dotty.tools.MainGenericCompiler.main(MainGenericCompiler.scala)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at coursier.bootstrap.launcher.a.a(Unknown Source)
	at coursier.bootstrap.launcher.Launcher.main(Unknown Source)

[bishabosha]

replacing the argument to resume results in another strange error:

trait SuspendSupport:
  type Suspension[-T, +R]
  extension [T, R](s: Suspension[T, R])
    def resume(arg: T): R

  def suspend[T, R](body: Suspension[T, R] => R): T

trait Async(using val support: SuspendSupport)

private class YZ(using ac: Async):

  private def test =
    Some(()).getOrElse:
      ac.support.suspend[scala.util.Try[Unit], Unit](k =>
        k.resume(??? : scala.util.Failure[Nothing])
      )

-- [E007] Type Mismatch Error: test.scala:17:8 ---------------------------------
17 |        k.resume(??? : scala.util.Failure[Nothing])
   |        ^
   |Found:    (k : YZ.this.ac.support.Suspension[util.Try[Unit], Unit])
   |Required: ?{ resume: ? }
   |Note that implicit extension methods cannot be applied because they are ambiguous;
   |both YZ.this.ac.support and YZ.this.ac.support provide an extension method `resume` on (k : YZ.this.ac.support.Suspension[util.Try[Unit], Unit])
   |
   | longer explanation available when compiling with `-explain`
1 error found

[bishabosha]

importing the extension explicitly avoids the crash, as there is no longer "implicit extension method" search:

trait SuspendSupport:
  type Suspension[-T, +R]

  extension [T, R](s: Suspension[T, R])
    def resume(arg: T): R

  def suspend[T, R](body: Suspension[T, R] => R): T

trait Async(using val support: SuspendSupport)

private class YZ(using ac: Async):
  import ac.support.resume

  private def test =
    Some(()).getOrElse:
      ac.support.suspend[scala.util.Try[Unit], Unit](k =>
        k.resume(scala.util.Failure(Exception()))
      )

[bishabosha]

This is also a regression from error to crash between 3.0.2 and 3.1.0 (fixing the usage of fewer braces) - however as it was always an error I think perhaps this isn't as serious as standard regression

[m8nmueller]

replacing the argument to resume results in another strange error:

trait SuspendSupport:
  type Suspension[-T, +R]
  extension [T, R](s: Suspension[T, R])
    def resume(arg: T): R

  def suspend[T, R](body: Suspension[T, R] => R): T

trait Async(using val support: SuspendSupport)

private class YZ(using ac: Async):

  private def test =
    Some(()).getOrElse:
      ac.support.suspend[scala.util.Try[Unit], Unit](k =>
        k.resume(??? : scala.util.Failure[Nothing])
      )

-- [E007] Type Mismatch Error: test.scala:17:8 ---------------------------------
17 |        k.resume(??? : scala.util.Failure[Nothing])
  |        ^
  |Found:    (k : YZ.this.ac.support.Suspension[util.Try[Unit], Unit])
  |Required: ?{ resume: ? }
  |Note that implicit extension methods cannot be applied because they are ambiguous;
  |both YZ.this.ac.support and YZ.this.ac.support provide an extension method `resume` on (k : YZ.this.ac.support.Suspension[util.Try[Unit], Unit])
  |
  | longer explanation available when compiling with `-explain`
1 error found

Note that this error (instead of the crash) can also be produced by unwrapping this code block outside of getOrElse.