Exception occurred while typechecking `summon` in nested `flatMap`

[armanbilge]

Compiler version

Scala compiler version 3.0.2 -- Copyright 2002-2021, LAMP/EPFL

Minimized code

trait Foo
trait Bar
object Foo:
  given (using Bar): Foo = ???

object Bug:
  def demonstrate: Unit =
    Option.empty[Unit].flatMap { _ =>
      Option.empty[Unit].map { _ =>
        val foo = summon[Foo]
        Option.empty[Unit]
      }
    }

Output (click arrow to expand)

exception occurred while typechecking bug.scala
exception occurred while compiling bug.scala
java.lang.AssertionError: assertion failed: Inconsistent state in TS[20X, 13, 1, 0]: it owns TypeVar(TypeParamRef(A)) whose owningState is TS[13, 1, 0] while compiling bug.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: Inconsistent state in TS[20X, 13, 1, 0]: it owns TypeVar(TypeParamRef(A)) whose owningState is TS[13, 1, 0]
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.core.TyperState.gc$$anonfun$1(TyperState.scala:220)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at dotty.tools.dotc.util.SimpleIdentitySet$Set1.foreach(SimpleIdentitySet.scala:69)
        at dotty.tools.dotc.core.TyperState.gc(TyperState.scala:226)
        at dotty.tools.dotc.core.TyperState.commit(TyperState.scala:163)
        at dotty.tools.dotc.typer.Inferencing$.isFullyDefined(Inferencing.scala:40)
        at dotty.tools.dotc.typer.Implicits.ignoredInstanceNormalImport$1(Implicits.scala:881)
        at dotty.tools.dotc.typer.Implicits.$anonfun$1(Implicits.scala:896)
        at dotty.tools.dotc.typer.ImplicitSearchError.hiddenImplicitsAddendum(ErrorReporting.scala:419)
        at dotty.tools.dotc.typer.ImplicitSearchError.missingArgMsg(ErrorReporting.scala:267)
        at dotty.tools.dotc.typer.Implicits.missingArgMsg(Implicits.scala:897)
        at dotty.tools.dotc.typer.Implicits.missingArgMsg$(Implicits.scala:774)
        at dotty.tools.dotc.typer.Typer.missingArgMsg(Typer.scala:106)
        at dotty.tools.dotc.typer.Typer.issueErrors$1$$anonfun$1$$anonfun$1(Typer.scala:3353)
        at dotty.tools.dotc.reporting.NoExplanation.msg(Message.scala:132)
        at dotty.tools.dotc.reporting.Message.message(Message.scala:87)
        at dotty.tools.dotc.reporting.Message.isNonSensical(Message.scala:99)
        at dotty.tools.dotc.reporting.HideNonSensicalMessages.isHidden(HideNonSensicalMessages.scala:16)
        at dotty.tools.dotc.reporting.HideNonSensicalMessages.isHidden$(HideNonSensicalMessages.scala:10)
        at dotty.tools.dotc.reporting.AbstractReporter.isHidden(AbstractReporter.scala:8)
        at dotty.tools.dotc.reporting.Reporter.report(Reporter.scala:148)
        at dotty.tools.dotc.reporting.Reporter.flush$$anonfun$1(Reporter.scala:206)
        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.reporting.Reporter.flush(Reporter.scala:206)
        at dotty.tools.dotc.core.TyperState.commit(TyperState.scala:143)
        at dotty.tools.dotc.typer.Applications.fail$1(Applications.scala:986)
        at dotty.tools.dotc.typer.Applications.realApply$7$$anonfun$6$$anonfun$2(Applications.scala:992)
        at scala.Option.getOrElse(Option.scala:201)
        at dotty.tools.dotc.typer.Applications.realApply$8$$anonfun$7(Applications.scala:993)
        at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:2952)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:994)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1032)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:317)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:106)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2693)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2756)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2821)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2825)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2941)
        at dotty.tools.dotc.typer.Typer.$anonfun$34(Typer.scala:2156)
        at dotty.tools.dotc.typer.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:225)
        at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2156)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2670)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2755)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2821)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2825)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2847)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2897)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2352)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$2(Typer.scala:2681)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2685)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2755)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2821)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2825)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2847)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2897)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2475)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2726)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2756)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2821)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2825)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2941)
        at dotty.tools.dotc.typer.FrontEnd.liftedTree1$1(FrontEnd.scala:79)
        at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:84)
        at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:43)
        at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:85)
        at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:120)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:333)
        at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:120)
        at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:205)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
        at dotty.tools.dotc.Run.runPhases$5(Run.scala:216)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:224)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:231)
        at dotty.tools.dotc.Run.compileSources(Run.scala:166)
        at dotty.tools.dotc.Run.compile(Run.scala:150)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
        at dotty.tools.dotc.Driver.process(Driver.scala:199)
        at dotty.tools.dotc.Driver.process(Driver.scala:167)
        at dotty.tools.dotc.Driver.process(Driver.scala:179)
        at dotty.tools.dotc.Driver.main(Driver.scala:209)
        at dotty.tools.dotc.Main.main(Main.scala)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:567)
        at coursier.bootstrap.launcher.a.a(Unknown Source)
        at coursier.bootstrap.launcher.Launcher.main(Unknown Source)

[armanbilge]

No problem when compiling with Scala compiler version 3.1.1-RC1-bin-20210908-8e92478-NIGHTLY-git-8e92478 -- Copyright 2002-2021, LAMP/EPFL. Can be closed?

[bishabosha]

Yeah can confirm on 3.1.0-RC1 we get the following output:

-- Error: sandbox/foo/i13497.scala:10:29 ---------------------------------------
10 |        val foo = summon[Foo]
   |                             ^
   |no implicit argument of type Foo was found for parameter x of method summon in object Predef.
   |I found:
   |
   |    Foo.given_Foo(/* missing */summon[Bar])
   |
   |But no implicit values were found that match type Bar.
1 error found

[bishabosha]

we can still add a regression test for it