Crash due to `wildApprox` failed to remove uninstantiated A` under new 3.6 implicits resolution

[WojciechMazur]

Based on OpenCB failure in takapi327/ldbc - build logs
The code is expected to fail compilation under 3.6 however, it should not crash. Related to #21212

Compiler version

3.6.0-RC1-bin-20240716-bbb45ca-NIGHTLY

Minimized code

trait FlatMap[F[_]]:
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B] = ???
  def ifM[B](ifTrue: => F[B], ifFalse: => F[B])(implicit F: FlatMap[F]): F[B] = ???
trait Monad[F[_]] extends FlatMap[F]
trait MonadError[F[_], E] extends Monad[F]:
  def raiseError[A](e: E): F[A]
trait Temporal[F[_]] extends MonadError[F, Throwable]

trait FlatMapOps[F[_], A]:
  def flatMap[B](f: A => F[B]): F[B] = ???
  def ifM[B](ifTrue: => F[B], ifFalse: => F[B])(implicit F: FlatMap[F]): F[B] = ???

implicit def toFlatMapOps[F[_], A](target: F[A])(implicit tc: FlatMap[F]): FlatMapOps[F, A] = ???

abstract class Ref[F[_], A]
object Ref:
  final class ApplyBuilders[F[_]]:
    def of[A](a: A): F[Ref[F, A]] = ???
  def apply[F[_]]: ApplyBuilders[F] = ???

trait DatabaseMetaData[F[_]]
class DatabaseMetaDataImpl[F[_]](
    statementClosed: Ref[F, Boolean],
    resultSetClosed: Ref[F, Boolean]
) extends DatabaseMetaData[F]

trait LdbcConnection[F[_]]:
  def getMetaData(): F[DatabaseMetaData[F]]

class ConnectionImpl[F[_]: Temporal](using ev: MonadError[F, Throwable])
    extends LdbcConnection[F]:
  def isClosed(): F[Boolean] = ???
  override def getMetaData(): F[DatabaseMetaData[F]] =
    isClosed().ifM(
      ev.raiseError(???),
      (for
        statementClosed <- Ref[F].of[Boolean](false)
        resultSetClosed <- Ref[F].of[Boolean](false)
      yield DatabaseMetaDataImpl[F](
        statementClosed,
        resultSetClosed
      ))
    )

Output (click arrow to expand)

[error] ./src/main/scala/test.scala:38:28
[error] Found:    F[Ref[F, Boolean]]
[error] Required: ?{ flatMap: ? }
[error] 
[error] where:    F is a type in class ConnectionImpl with bounds <: [_] =>> Any
[error] 
[error] Note that implicit conversions cannot be applied because they are ambiguous;
[error] both value evidence$1 in class ConnectionImpl and given instance ev in class ConnectionImpl match type FlatMap[F]
[error]         statementClosed <- Ref[F].of[Boolean](false)
[error]                            ^^^^^^^^^^^^^^^^^^^^^^^^^

  exception occurred while typechecking /Users/wmazur/projects/sandbox/src/main/scala/test.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: /Users/wmazur/projects/sandbox/src/main/scala/test.scala
        during phase: typer
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.14
    compiler version: version 3.6.0-RC1-bin-20240716-bbb45ca-NIGHTLY-git-bbb45ca
            settings: -bootclasspath /Users/wmazur/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala3-library_3/3.6.0-RC1-bin-20240716-bbb45ca-NIGHTLY/scala3-library_3-3.6.0-RC1-bin-20240716-bbb45ca-NIGHTLY.jar:/Users/wmazur/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.14/scala-library-2.13.14.jar -classpath /Users/wmazur/projects/sandbox/src/.scala-build/.bloop/src_aea078f50b/bloop-internal-classes/main-q0_2TGcRQVKPA6upcOSarA==:/Users/wmazur/projects/sandbox/src/.scala-build/src_aea078f50b/classes/classes-empty-src_aea078f50b -d /Users/wmazur/projects/sandbox/src/.scala-build/.bloop/src_aea078f50b/bloop-internal-classes/main-q0_2TGcRQVKPA6upcOSarA== -java-output-version 17 -sourceroot /Users/wmazur/projects/sandbox/src

Error compiling project (Scala 3.6.0-RC1-bin-20240716-bbb45ca-NIGHTLY, JVM (17))
Error: Unexpected error when compiling src_aea078f50b: java.lang.AssertionError: assertion failed: `wildApprox` failed to remove uninstantiated A
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:667)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6807)
        at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6880)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:675)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy26$1(Types.scala:6790)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$3(Types.scala:6790)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6794)
        at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6880)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:675)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6872)
        at dotty.tools.dotc.typer.ProtoTypes$FunProto.fold(ProtoTypes.scala:603)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6842)
        at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6880)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:675)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.typer.ProtoTypes$SelectionProto.fold(ProtoTypes.scala:279)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6842)
        at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6880)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:675)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:6879)
        at dotty.tools.dotc.typer.ProtoTypes$ViewProto.fold(ProtoTypes.scala:660)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6842)
        at dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:6880)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.traverse(Implicits.scala:675)
        at dotty.tools.dotc.typer.ImplicitRunInfo$collectParts$2$.apply(Implicits.scala:681)
        at dotty.tools.dotc.typer.ImplicitRunInfo.recur$1(Implicits.scala:757)
        at dotty.tools.dotc.typer.ImplicitRunInfo.computeIScope(Implicits.scala:768)
        at dotty.tools.dotc.typer.ImplicitRunInfo.$anonfun$1(Implicits.scala:844)
        at dotty.tools.dotc.util.ReadOnlyMap.getOrElse(ReadOnlyMap.scala:23)
        at dotty.tools.dotc.typer.ImplicitRunInfo.implicitScope(Implicits.scala:844)
        at dotty.tools.dotc.typer.ImplicitRunInfo.implicitScope$(Implicits.scala:619)
        at dotty.tools.dotc.Run.implicitScope(Run.scala:43)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.implicitScope(Implicits.scala:1762)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1647)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit$$anonfun$3(Implicits.scala:1731)
        at dotty.tools.dotc.typer.Implicits$SearchResult.recoverWith(Implicits.scala:430)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1717)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.bestImplicit(Implicits.scala:1758)
        at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1104)
        at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:855)
        at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:145)
        at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1124)
        at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:855)
        at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:145)
        at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:1124)
        at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:855)
        at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:145)
        at dotty.tools.dotc.typer.Implicits.inferView(Implicits.scala:895)
        at dotty.tools.dotc.typer.Implicits.inferView$(Implicits.scala:855)
        at dotty.tools.dotc.typer.Typer.inferView(Typer.scala:145)
        at dotty.tools.dotc.typer.Typer.tryExtensionOrConversion(Typer.scala:3915)
        at dotty.tools.dotc.typer.Typer.tryExt$1(Typer.scala:800)
        at dotty.tools.dotc.typer.Typer.typedSelectWithAdapt(Typer.scala:887)
        at dotty.tools.dotc.typer.Typer.typeSelectOnTerm$1(Typer.scala:978)
        at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:1016)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3410)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3519)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3597)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3601)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3712)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1045)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1236)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:434)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:145)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3435)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3520)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3597)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3601)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3712)
        at dotty.tools.dotc.typer.Typer.$anonfun$65(Typer.scala:2859)
        at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:256)
        at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2859)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3417)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3519)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3597)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3601)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3623)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3669)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:3117)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:3423)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:3427)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3519)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3597)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3601)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3623)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3669)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:3250)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:3469)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:3520)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3597)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3601)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3712)
        at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$1(TyperPhase.scala:47)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:503)
        at dotty.tools.dotc.typer.TyperPhase.typeCheck(TyperPhase.scala:53)
        at dotty.tools.dotc.typer.TyperPhase.$anonfun$4(TyperPhase.scala:99)
        at scala.collection.Iterator$$anon$6.hasNext(Iterator.scala:479)
        at scala.collection.Iterator$$anon$9.hasNext(Iterator.scala:583)
        at scala.collection.immutable.List.prependedAll(List.scala:152)
        at scala.collection.immutable.List$.from(List.scala:685)
        at scala.collection.immutable.List$.from(List.scala:682)
        at scala.collection.IterableOps$WithFilter.map(Iterable.scala:900)
        at dotty.tools.dotc.typer.TyperPhase.runOn(TyperPhase.scala:98)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:384)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:396)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:396)
        at dotty.tools.dotc.Run.compileSources(Run.scala:282)
        at dotty.tools.dotc.Run.compile(Run.scala:267)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
        at dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:141)
        at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
        at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.compileSources$1(BloopHighLevelCompiler.scala:133)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$7(BloopHighLevelCompiler.scala:159)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$1(BloopHighLevelCompiler.scala:71)
        at bloop.tracing.NoopTracer$.trace(BraveTracer.scala:53)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.timed$1(BloopHighLevelCompiler.scala:70)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$6(BloopHighLevelCompiler.scala:159)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at monix.eval.internal.TaskRunLoop$.startFull(TaskRunLoop.scala:81)
        at monix.eval.internal.TaskRestartCallback.syncOnSuccess(TaskRestartCallback.scala:101)
        at monix.eval.internal.TaskRestartCallback.onSuccess(TaskRestartCallback.scala:74)
        at monix.eval.internal.TaskExecuteOn$AsyncRegister$$anon$1.run(TaskExecuteOn.scala:71)
        at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)

Output under -source:3.5

[warn] ./src/main/scala/test.scala:41:6
[warn] Given search preference for Monad[F] between alternatives (ConnectionImpl.this.evidence$1 : Temporal[F]) and (ConnectionImpl.this.ev : MonadError[F, Throwable]) will change
[warn] Current choice           : the first alternative
[warn] New choice from Scala 3.6: none - it's ambiguous
[warn] 
[warn] where:    F is a type in class ConnectionImpl with bounds <: [_] =>> Any
[warn]     )
[warn]      ^
[warn] ./src/main/scala/test.scala:39:53
[warn] Given search preference for Monad[F] between alternatives (ConnectionImpl.this.evidence$1 : Temporal[F]) and (ConnectionImpl.this.ev : MonadError[F, Throwable]) will change
[warn] Current choice           : the first alternative
[warn] New choice from Scala 3.6: none - it's ambiguous
[warn] 
[warn] where:    F is a type in class ConnectionImpl with bounds <: [_] =>> Any
[warn]         resultSetClosed <- Ref[F].of[Boolean](false)
[warn]                                                     ^
[warn] ./src/main/scala/test.scala:38:53
[warn] Given search preference for Monad[F] between alternatives (ConnectionImpl.this.evidence$1 : Temporal[F]) and (ConnectionImpl.this.ev : MonadError[F, Throwable]) will change
[warn] Current choice           : the first alternative
[warn] New choice from Scala 3.6: none - it's ambiguous
[warn] 
[warn] where:    F is a type in class ConnectionImpl with bounds <: [_] =>> Any
[warn]         statementClosed <- Ref[F].of[Boolean](false)
[warn]                                                     ^
[warn] ./src/main/scala/test.scala:35:15
[warn] Given search preference for Monad[F] between alternatives (ConnectionImpl.this.evidence$1 : Temporal[F]) and (ConnectionImpl.this.ev : MonadError[F, Throwable]) will change
[warn] Current choice           : the first alternative
[warn] New choice from Scala 3.6: none - it's ambiguous
[warn] 
[warn] where:    F is a type in class ConnectionImpl with bounds <: [_] =>> Any
[warn]     isClosed().ifM(

[WojciechMazur]

Fixed probably in #21226 . No longer crashes in 3.6.0-RC1-bin-20240719-af933c4-NIGHTLY

[EugeneFlesselle]

@WojciechMazur was line 38 statementClosed <- Ref[F].of[Boolean](false) the only expected error ?

[EugeneFlesselle]

@WojciechMazur was line 38 statementClosed <- Ref[F].of[Boolean](false) the only expected error ?

My bad, line 39 resultSetClosed <- Ref[F].of[Boolean](false)

[WojciechMazur]

The reproducer was missing a single line with a map method

trait FlatMapOps[F[_], A]:
  def map[B](f: A => B): F[B] = ???

With line present it would compile without errors, but would crash in in 3.5 + -source:3.6 or 3.6+