Illegal MatchType Error reaches the backend #19434

odersky · 2024-01-12T17:24:18Z

Compiler version

3.4.0

Minimized example

object Test:

  object Named:
    opaque type Named[name <: String & Singleton, A] >: A = A

  type DropNames[T <: Tuple] = T match
    case Named.Named[_, x] *: xs => x *: DropNames[xs]
    case _ => T

  def f[T <: Tuple]: DropNames[T] = ???

Output

an unexpected type representation reached the compiler backend while compiling named-tuples-strawman.scala: dotty.tools.dotc.core.Types$PreviousErrorType@2d9dff65. If possible, please file a bug on https://github.com/lampepfl/dotty/issues
scala.MatchError: dotty.tools.dotc.core.Types$PreviousErrorType@2d9dff65 (of class dotty.tools.dotc.core.Types$PreviousErrorType)
	at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$typeToTypeKind(BCodeHelpers.scala:764)
	at dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.toTypeKind(BCodeHelpers.scala:202)
	at dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.toTypeKind$(BCodeHelpers.scala:130)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.toTypeKind(BCodeSkelBuilder.scala:134)
	at dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.asmMethodType(BCodeHelpers.scala:188)
	at dotty.tools.backend.jvm.BCodeHelpers$BCInnerClassGen.asmMethodType$(BCodeHelpers.scala:130)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.asmMethodType(BCodeSkelBuilder.scala:134)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:815)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:693)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen$$anonfun$1(BCodeSkelBuilder.scala:699)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:699)
	at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:293)
	at dotty.tools.backend.jvm.CodeGen.genClass(CodeGen.scala:154)
	at dotty.tools.backend.jvm.CodeGen.genClassDef$1(CodeGen.scala:62)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1(CodeGen.scala:117)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1$$anonfun$1(CodeGen.scala:115)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.backend.jvm.CodeGen.genClassDefs$1(CodeGen.scala:115)
	at dotty.tools.backend.jvm.CodeGen.genUnit(CodeGen.scala:120)
	at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:83)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:354)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:360)
	at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:91)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:315)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:337)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:348)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:357)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:357)
	at dotty.tools.dotc.Run.compileSources(Run.scala:261)
	at dotty.tools.dotc.Run.compile(Run.scala:246)
	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.dotc.Main.main(Main.scala)
Error while emitting named-tuples-strawman.scala
dotty.tools.dotc.core.Types$PreviousErrorType@2d9dff65 (of class dotty.tools.dotc.core.Types$PreviousErrorType)
1 warning found
1 error found

Expectation

A regular error, like this one:

1 |object Test:
  |       ^
  |       The match type contains an illegal case:
  |           case Test.Named.Named[_, x] *: xs => x *: Test.DropNames[xs]
  |       (this error can be ignored for now with `-source:3.3`)

I verified that that was the error that generated the illegal error type.

The text was updated successfully, but these errors were encountered:

odersky · 2024-01-12T17:32:36Z

Also, I am not sure why this program would be illegal. I guess it should not be.

The problem seems to be that the match type is reduced only at erasure. At that time, opaque types are already eliminated, so Named looks like a normal type alias. So the type comparer complains.

odersky · 2024-01-12T17:36:13Z

Here's the stacktrace I see when I add a new Error().printStackTrace() to line 3573 of TypeComparer where the ErrorType gets generated:

	at dotty.tools.dotc.core.TrackingTypeComparer.recur$2(TypeComparer.scala:3574)
	at dotty.tools.dotc.core.TrackingTypeComparer.op$proxy82$1(TypeComparer.scala:3614)
	at dotty.tools.dotc.core.TrackingTypeComparer.matchCases(TypeComparer.scala:3615)
	at dotty.tools.dotc.core.Types$MatchType.matchCases$1(Types.scala:5041)
	at dotty.tools.dotc.core.Types$MatchType.reduced$$anonfun$1(Types.scala:5050)
	at dotty.tools.dotc.core.TypeComparer.inSubComparer(TypeComparer.scala:3054)
	at dotty.tools.dotc.core.TypeComparer.tracked(TypeComparer.scala:3064)
	at dotty.tools.dotc.core.TypeComparer$.tracked(TypeComparer.scala:3232)
	at dotty.tools.dotc.core.Types$MatchType.reduced(Types.scala:5050)
	at dotty.tools.dotc.core.Types$MatchType.tryNormalize(Types.scala:4999)
	at dotty.tools.dotc.core.Types$AppliedType.tryMatchAlias$1$$anonfun$1(Types.scala:4562)
	at dotty.tools.dotc.core.MatchTypeTrace$.recurseWith(MatchTypeTrace.scala:81)
	at dotty.tools.dotc.core.Types$AppliedType.tryMatchAlias$1(Types.scala:4563)
	at dotty.tools.dotc.core.Types$AppliedType.tryNormalize(Types.scala:4568)
	at dotty.tools.dotc.core.Types$AppliedType.translucentSuperType(Types.scala:4541)
	at dotty.tools.dotc.core.TypeErasure.checkedSuperType(TypeErasure.scala:752)
	at dotty.tools.dotc.core.TypeErasure.dotty$tools$dotc$core$TypeErasure$$apply(TypeErasure.scala:642)
	at dotty.tools.dotc.core.TypeErasure.eraseResult(TypeErasure.scala:879)
	at dotty.tools.dotc.core.TypeErasure.eraseInfo(TypeErasure.scala:815)
	at dotty.tools.dotc.core.TypeErasure$.transformInfo(TypeErasure.scala:273)
	at dotty.tools.dotc.transform.Erasure.transform(Erasure.scala:96)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.goForward$1(Denotations.scala:831)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.current(Denotations.scala:877)
	at dotty.tools.dotc.core.Symbols$Symbol.recomputeDenot(Symbols.scala:124)
	at dotty.tools.dotc.core.Symbols$Symbol.computeDenot(Symbols.scala:118)
	at dotty.tools.dotc.core.Symbols$Symbol.denot(Symbols.scala:111)
	at dotty.tools.dotc.core.Symbols$.toDenot(Symbols.scala:520)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.paramAccessors$$anonfun$1(SymDenotations.scala:2386)
	at dotty.tools.dotc.core.Scopes$Scope.filter(Scopes.scala:103)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.paramAccessors(SymDenotations.scala:2386)
	at dotty.tools.dotc.core.Contexts$Context.superCallContext(Contexts.scala:417)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2720)
	at dotty.tools.dotc.transform.Erasure$Typer.typedClassDef(Erasure.scala:1047)

sjrd · 2024-01-23T08:56:20Z

Definitely a problem in erasure. During erasure, when trying to reduce the match type before erasing its bound, I observe a weird behavior. It sees as tycon of the match case pattern Test.Named.Named -- so far so good. But tycon.info gives [name <: String & Singleton,A] = A. That does not seem correct to me. At erasure, the info should still be the public abstract bounds >: [name] =>> Tupe <: [name] =>> Any. Shouldn't it? ctx.phase is erasure when it computes that.

odersky · 2024-01-23T09:09:18Z

No, opaque type aliases are eliminated before erasure, at phase ElimOpaque.

sjrd · 2024-01-23T09:12:58Z

Oh. Well at least that explains it. It will make this tricky to fix, though. I'll have to think about it more.

odersky · 2024-02-01T12:33:45Z

I just hit this in the wild with some test code using NamedTuples.

dwijnand · 2024-02-01T12:44:57Z

I'm merging ElimOpaque into Erasure

This is another case which used to result in an unreported `ErrorType` It is now detected only when enabling `-Yforce-sbt-phases -Xverify-signatures -Ycheck:all` The error comes from: ```scala type Names[X <: AnyNamedTuple] <: Tuple = X match case NamedTuple[n, _] => n ``` The `NamedTuple` pattern is legal as long as the type alias is opaque. Following elimOpaque however, it is beta-reduced (as normal applied type aliases in match type patterns) to a pattern which no longer contains an instance of the type capture `n`, making it an illegal pattern. This used to return an `ErrorType`, which happened not be reported as in the other cases. Also note this error could also be missed without the New footprint calculation scheme (scala#19639) Given the changes to the footprint calculation and failed reduction reporting, this is now the same issue as scala#19434.

odersky added stat:needs triage Every issue needs to have an "area" and "itype" label itype:bug area:match-types and removed stat:needs triage Every issue needs to have an "area" and "itype" label labels Jan 12, 2024

odersky assigned sjrd Jan 12, 2024

dwijnand mentioned this issue Jan 26, 2024

Guard against opaques crashing the compiler #19541

Closed

This was referenced Mar 25, 2024

Update unreducible match types error reporting #19954

Merged

Do match type reduction atPhaseNoLater than ElimOpaque #20017

Merged

EugeneFlesselle closed this as completed in #20017 Mar 27, 2024

Kordyjan added this to the 3.5.0 milestone May 10, 2024

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Illegal MatchType Error reaches the backend #19434

Illegal MatchType Error reaches the backend #19434

odersky commented Jan 12, 2024 •

edited

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odersky commented Jan 12, 2024

odersky commented Jan 12, 2024

sjrd commented Jan 23, 2024

odersky commented Jan 23, 2024

sjrd commented Jan 23, 2024

odersky commented Feb 1, 2024

dwijnand commented Feb 1, 2024

Illegal MatchType Error reaches the backend #19434

Illegal MatchType Error reaches the backend #19434

Comments

odersky commented Jan 12, 2024 • edited Loading

Compiler version

Minimized example

Output

Expectation

odersky commented Jan 12, 2024

odersky commented Jan 12, 2024

sjrd commented Jan 23, 2024

odersky commented Jan 23, 2024

sjrd commented Jan 23, 2024

odersky commented Feb 1, 2024

dwijnand commented Feb 1, 2024

odersky commented Jan 12, 2024 •

edited

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