Compiler crash when deriving enums

[WojciechMazur]

Compiler version

3.1.2
Fails with 3.2.0-RC1-bin-20220512-9dbe2d3-NIGHTLY
Works with 3.1.1

Minimized code

Based on:

• https://github.com/jcouyang/dhall-generic/blob/2ad4bddb18ff43bb186644ba4999167e71ace8ef/src/main/scala-3/us/oyanglul/dhall/generic.scala
• https://github.com/jcouyang/dhall-generic/blob/69875512e8269a51ee4d11d46f3168a0dfb16471/src/test/scala-3/us/oyanglul/dhall/Data.scala

decoder.scala

import scala.deriving._

sealed trait Decoder[T]
object Decoder {
  given Decoder[Double] = ???

  inline given summonEmptyTuple[H]: Tuple.Map[EmptyTuple.type, Decoder] =
    EmptyTuple

  inline given summonTuple[H, T <: Tuple](using hd: Decoder[H], td: Tuple.Map[T, Decoder]): Tuple.Map[H *: T, Decoder] =
    hd *: td

  inline given derived[T](using m: Mirror.Of[T], d: Tuple.Map[m.MirroredElemTypes, Decoder]): Decoder[T] = ???
}

test.scala

//> using scala "3.1.1"
// Fails in each cases below
enum Env derives Decoder:
  case Local,Sit,Prod

enum Env2 derives Decoder:
  case Local()
  case Sit()
  case Prod()

enum Shape derives Decoder:
  case Rectangle(width: Double, height: Double)
  case Circle(radius: Double)

decoder.scala and test.scala needs to be compiled seperetly

scalac decoder.scala
scalac test.scala

Output (click arrow to expand)

➜  dotty git:(main) ✗ ./bin/scalac test.scala 
exception occurred while typechecking test.scala
exception occurred while compiling test.scala
java.lang.NullPointerException while compiling test.scala
Exception in thread "main" java.lang.NullPointerException
        at dotty.tools.dotc.core.TypeOps$.simplify(TypeOps.scala:148)
        at dotty.tools.dotc.core.Types$Type.simplified(Types.scala:1894)
        at dotty.tools.dotc.core.ConstraintHandling.instanceType(ConstraintHandling.scala:557)
        at dotty.tools.dotc.core.ConstraintHandling.instanceType$(ConstraintHandling.scala:27)
        at dotty.tools.dotc.core.TypeComparer.instanceType(TypeComparer.scala:30)
        at dotty.tools.dotc.core.TypeComparer$.instanceType(TypeComparer.scala:2817)
        at dotty.tools.dotc.core.Types$TypeVar.instantiate(Types.scala:4753)
        at dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.instantiate(Inferencing.scala:167)
        at dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:189)
        at dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:174)
        at dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy23$1(Types.scala:6019)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$3(Types.scala:6019)
        at dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:6023)
        at dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:199)
        at dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.process(Inferencing.scala:210)
        at dotty.tools.dotc.typer.Inferencing$.isFullyDefined(Inferencing.scala:34)
        at dotty.tools.dotc.core.TypeOps$.simplify(TypeOps.scala:142)
        at dotty.tools.dotc.core.TypeOps$.mapArgs$1(TypeOps.scala:145)
        at dotty.tools.dotc.core.TypeOps$.mapArgs$1(TypeOps.scala:145)
        at dotty.tools.dotc.core.TypeOps$.simplify(TypeOps.scala:145)
        at dotty.tools.dotc.core.Types$Type.simplified(Types.scala:1894)
        at dotty.tools.dotc.core.TrackingTypeComparer.matchCase$1(TypeComparer.scala:2932)
        at dotty.tools.dotc.core.TrackingTypeComparer.recur$2(TypeComparer.scala:2947)
        at dotty.tools.dotc.core.TrackingTypeComparer.op$proxy73$1(TypeComparer.scala:2977)
        at dotty.tools.dotc.core.TrackingTypeComparer.matchCases(TypeComparer.scala:2978)
        at dotty.tools.dotc.core.Types$MatchType.matchCases$1(Types.scala:4870)
        at dotty.tools.dotc.core.Types$MatchType.reduced$$anonfun$1(Types.scala:4879)
        at dotty.tools.dotc.core.TypeComparer.inSubComparer(TypeComparer.scala:2683)
        at dotty.tools.dotc.core.TypeComparer.tracked(TypeComparer.scala:2693)
        at dotty.tools.dotc.core.TypeComparer$.tracked(TypeComparer.scala:2850)
        at dotty.tools.dotc.core.Types$MatchType.reduced(Types.scala:4879)
        at dotty.tools.dotc.core.Types$MatchType.tryNormalize(Types.scala:4828)
        at dotty.tools.dotc.core.Types$Type.normalized(Types.scala:1430)
        at dotty.tools.dotc.core.Types$AppliedType.superType(Types.scala:4161)
        at dotty.tools.dotc.core.TypeComparer.$anonfun$4(TypeComparer.scala:1120)
        at scala.Function0.apply$mcZ$sp(Function0.scala:39)
        at dotty.tools.dotc.core.TypeComparer.tryAlso(TypeComparer.scala:1669)
        at dotty.tools.dotc.core.TypeComparer.loop$1(TypeComparer.scala:1120)
        at dotty.tools.dotc.core.TypeComparer.isMatchingApply$1(TypeComparer.scala:1137)
        at dotty.tools.dotc.core.TypeComparer.compareAppliedType2$1(TypeComparer.scala:1207)
        at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:571)
        at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:504)
        at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:381)
        at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1335)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:192)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:202)
        at dotty.tools.dotc.core.TypeComparer.topLevelSubType(TypeComparer.scala:126)
        at dotty.tools.dotc.core.TypeComparer.necessarySubType(TypeComparer.scala:137)
        at dotty.tools.dotc.core.TypeComparer$.necessarySubType(TypeComparer.scala:2750)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.necessarilyCompatible(ProtoTypes.scala:46)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.necessarilyCompatible$(ProtoTypes.scala:25)
        at dotty.tools.dotc.typer.Typer.necessarilyCompatible(Typer.scala:117)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.constrainResult(ProtoTypes.scala:96)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.constrainResult$(ProtoTypes.scala:25)
        at dotty.tools.dotc.typer.Typer.constrainResult(Typer.scala:117)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.constrainResult(ProtoTypes.scala:113)
        at dotty.tools.dotc.typer.ProtoTypes$Compatibility.constrainResult$(ProtoTypes.scala:25)
        at dotty.tools.dotc.typer.Typer.constrainResult(Typer.scala:117)
        at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:3807)
        at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:4043)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:3377)
        at dotty.tools.dotc.typer.Typer.readapt$1(Typer.scala:3388)
        at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:4030)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:3377)
        at dotty.tools.dotc.typer.Implicits.typedImplicit(Implicits.scala:1053)
        at dotty.tools.dotc.typer.Implicits.typedImplicit$(Implicits.scala:785)
        at dotty.tools.dotc.typer.Typer.typedImplicit(Typer.scala:117)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.tryImplicit(Implicits.scala:1173)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.rank$1(Implicits.scala:1272)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1442)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1470)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.searchImplicit(Implicits.scala:1478)
        at dotty.tools.dotc.typer.Implicits$ImplicitSearch.bestImplicit(Implicits.scala:1503)
        at dotty.tools.dotc.typer.Implicits.inferImplicit(Implicits.scala:997)
        at dotty.tools.dotc.typer.Implicits.inferImplicit$(Implicits.scala:785)
        at dotty.tools.dotc.typer.Typer.inferImplicit(Typer.scala:117)
        at dotty.tools.dotc.typer.Implicits.inferImplicitArg(Implicits.scala:851)
        at dotty.tools.dotc.typer.Implicits.inferImplicitArg$(Implicits.scala:785)
        at dotty.tools.dotc.typer.Typer.inferImplicitArg(Typer.scala:117)
        at dotty.tools.dotc.typer.Typer.implicitArgs$1(Typer.scala:3499)
        at dotty.tools.dotc.typer.Typer.inferArgsAfter$1(Typer.scala:3497)
        at dotty.tools.dotc.typer.Typer.implicitArgs$1(Typer.scala:3532)
        at dotty.tools.dotc.typer.Typer.addImplicitArgs$1(Typer.scala:3535)
        at dotty.tools.dotc.typer.Typer.adaptNoArgsImplicitMethod$1(Typer.scala:3611)
        at dotty.tools.dotc.typer.Typer.adaptNoArgs$1(Typer.scala:3809)
        at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:4043)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:3377)
        at dotty.tools.dotc.typer.Typer.readapt$1(Typer.scala:3388)
        at dotty.tools.dotc.typer.Typer.adapt1(Typer.scala:4030)
        at dotty.tools.dotc.typer.Typer.adapt(Typer.scala:3377)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2994)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2998)
        at dotty.tools.dotc.typer.Deriving$Deriver.typeclassInstance$1$$anonfun$1(Deriving.scala:295)
        at dotty.tools.dotc.typer.Deriving$Deriver.syntheticDef$1(Deriving.scala:301)
        at dotty.tools.dotc.typer.Deriving$Deriver.syntheticDefs$$anonfun$1(Deriving.scala:304)
        at scala.collection.StrictOptimizedIterableOps.map(StrictOptimizedIterableOps.scala:100)
        at scala.collection.StrictOptimizedIterableOps.map$(StrictOptimizedIterableOps.scala:87)
        at scala.collection.mutable.ListBuffer.map(ListBuffer.scala:39)
        at dotty.tools.dotc.typer.Deriving$Deriver.syntheticDefs(Deriving.scala:304)
        at dotty.tools.dotc.typer.Deriving$Deriver.finalize(Deriving.scala:309)
        at dotty.tools.dotc.typer.Typer.finalize$1(Typer.scala:3064)
        at dotty.tools.dotc.typer.Typer.$anonfun$65(Typer.scala:3071)
        at scala.collection.immutable.List.mapConserve(List.scala:472)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3071)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2636)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2898)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2929)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2994)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2998)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3114)
        at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$1(TyperPhase.scala:43)
        at dotty.tools.dotc.typer.TyperPhase.typeCheck$$anonfun$adapted$1(TyperPhase.scala:50)
        at scala.Function0.apply$mcV$sp(Function0.scala:39)
        at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:414)
        at dotty.tools.dotc.typer.TyperPhase.typeCheck(TyperPhase.scala:50)
        at dotty.tools.dotc.typer.TyperPhase.runOn$$anonfun$3(TyperPhase.scala:84)
        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:84)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:225)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:236)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:244)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:253)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:253)
        at dotty.tools.dotc.Run.compileSources(Run.scala:186)
        at dotty.tools.dotc.Run.compile(Run.scala:170)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
        at dotty.tools.dotc.Driver.process(Driver.scala:195)
        at dotty.tools.dotc.Driver.process(Driver.scala:163)
        at dotty.tools.dotc.Driver.process(Driver.scala:175)
        at dotty.tools.dotc.Driver.main(Driver.scala:205)
        at dotty.tools.dotc.Main.main(Main.scala)

[griggt]

This errors (but does not crash) on 3.1.2. The errors started with 3ab18a9.

The crashing started with #15036, which was backported to 3.1.3-RC3, so it crashes there as well.

[prolativ]

Minimized further:

//> using scala "3.2.0-RC1-bin-20220512-9dbe2d3-NIGHTLY"

import scala.deriving._

enum Env:
  case Local,Sit,Prod

sealed trait Decoder[T]
object Decoder:
  given summonTuple[H, T <: Tuple]: Tuple.Map[H *: T, Decoder] = ???
  def derived(using m: Mirror.Of[Env], d: Tuple.Map[m.MirroredElemTypes, Decoder]): Decoder[Env] = ???

val x = Decoder.derived

[griggt]

This appears to have been fixed by a combination of #15310 (the NPE) and #15036 (the match type reduction failure).

It still crashes on 3.1.3-RC4 however. Is it too late to backport #15310?