AssertionError: invalid prefix MethodType ...

[mateusz-lichota]

Compiler version

3.2.2

Minimized code

sealed trait Expr1
sealed trait Literal extends Expr1

case class ArrayLiter(elems: List[Expr1]) extends Literal

sealed trait SemanticType {
  type T // the type with which a literal of this semanticType is represented
}
case object SemanticInt extends SemanticType {
  type T = Int
}

case class SemanticArray[U <: SemanticType](dim: Int) extends SemanticType {
  type T = List[U]
}

sealed trait Expr2[+T]
class Liter[T <: SemanticType](val ty: T, val value: ty.T) extends Expr2[T]

def typecheckArrayLiter(
    a: ArrayLiter
): Liter[SemanticArray[SemanticType]] = {
  val x: List[Expr2[SemanticInt.type]] = List()
  Liter(SemanticArray[SemanticInt.type], x)
}

Output (click arrow to expand)

java.lang.AssertionError: assertion failed: invalid prefix MethodType(List(dim), List(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int)), AppliedType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class SemanticArray),List(TermRef(ThisType(TypeRef(NoPrefix,module class <empty>)),object SemanticInt)))) while typechecking src/main/scala/wacc/Main.scala
exception occurred while typechecking src/main/scala/wacc/Main.scala
exception occurred while compiling src/main/scala/wacc/Main.scala
java.lang.AssertionError: assertion failed: invalid prefix MethodType(List(dim), List(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int)), AppliedType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class SemanticArray),List(TermRef(ThisType(TypeRef(NoPrefix,module class <empty>)),object SemanticInt)))) while compiling src/main/scala/wacc/Main.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: invalid prefix MethodType(List(dim), List(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int)), AppliedType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class SemanticArray),List(TermRef(ThisType(TypeRef(NoPrefix,module class <empty>)),object SemanticInt))))
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.core.Types$NamedType.<init>(Types.scala:2189)
        at dotty.tools.dotc.core.Types$TypeRef.<init>(Types.scala:2804)
        at dotty.tools.dotc.core.Types$CachedTypeRef.<init>(Types.scala:2860)
        at dotty.tools.dotc.core.Uniques$NamedTypeUniques.newType$1(Uniques.scala:45)
        at dotty.tools.dotc.core.Uniques$NamedTypeUniques.enterIfNew(Uniques.scala:46)
        at dotty.tools.dotc.core.Types$TypeRef$.apply(Types.scala:2919)
        at dotty.tools.dotc.core.Types$TypeRef$.apply(Types.scala:2925)
        at dotty.tools.dotc.core.Types$NamedType$.apply(Types.scala:2899)
        at dotty.tools.dotc.core.Types$NamedType.reload$1(Types.scala:2706)
        at dotty.tools.dotc.core.Types$NamedType.withPrefix(Types.scala:2722)
        at dotty.tools.dotc.core.Types$NamedType.derivedSelect(Types.scala:2636)
        at dotty.tools.dotc.core.Substituters$.substParam(Substituters.scala:142)
        at dotty.tools.dotc.core.Types$Type.substParam(Types.scala:1813)
        at dotty.tools.dotc.typer.TypeAssigner.safeSubstParam(TypeAssigner.scala:275)
        at dotty.tools.dotc.typer.TypeAssigner.safeSubstParam$(TypeAssigner.scala:16)
        at dotty.tools.dotc.typer.Typer.safeSubstParam(Typer.scala:121)
        at dotty.tools.dotc.typer.Applications$Application.addTyped$1$$anonfun$1(Applications.scala:579)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:91)
        at dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:107)
        at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:579)
        at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:640)
        at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:479)
        at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:766)
        at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:883)
        at dotty.tools.dotc.typer.Applications.ApplyTo(Applications.scala:1095)
        at dotty.tools.dotc.typer.Applications.ApplyTo$(Applications.scala:346)
        at dotty.tools.dotc.typer.Typer.ApplyTo(Typer.scala:121)
        at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:941)
        at dotty.tools.dotc.typer.Applications.realApply$1$$anonfun$3(Applications.scala:1021)
        at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:3169)
        at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:1032)
        at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:1070)
        at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:346)
        at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:121)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2895)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2958)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3025)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3145)
        at dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:1069)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2903)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2958)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3025)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3145)
        at dotty.tools.dotc.typer.Typer.$anonfun$51(Typer.scala:2356)
        at dotty.tools.dotc.inlines.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:249)
        at dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:2356)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2871)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2957)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3025)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3051)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3101)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2532)
        at dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$1(Typer.scala:2883)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2887)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2957)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3025)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:3051)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:3101)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2659)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2928)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2958)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3025)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:3029)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:3145)
        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:429)
        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$1(Run.scala:238)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:249)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:257)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:266)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:266)
        at dotty.tools.dotc.Run.compileSources(Run.scala:190)
        at dotty.tools.dotc.Run.compile(Run.scala:174)
        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.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:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at coursier.bootstrap.launcher.a.a(Unknown Source)
        at coursier.bootstrap.launcher.Launcher.main(Unknown Source)

[jchyb]

Hi! Thank you for submitting the issue. Fortunately, this appears to be fixed on main (21729d2) and in 3.3.0-RC2, which means that this will be fixed in the next version of Scala (3.3.0). Instead of a crash we now get errors, all of which appear to be correct:

-- [E007] Type Mismatch Error: main.scala:24:8 ---------------------------------
24 |  Liter(SemanticArray[SemanticInt.type], x)
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |        Found:    Int => SemanticArray[SemanticInt.type]
   |        Required: SemanticArray[SemanticType]
   |
   | longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: main.scala:24:41 --------------------------------
24 |  Liter(SemanticArray[SemanticInt.type], x)
   |                                         ^
   |Found:    (x : List[Expr2[SemanticInt.type]])
   |Required: ?1.T
   |
   |where:    ?1 is an unknown value of type (dim: Int): SemanticArray[SemanticInt.type]
   |
   | longer explanation available when compiling with `-explain`

As an extra, here is a version that fixes the above errors (and also compiles on 3.2.2):

sealed trait Expr1
sealed trait Literal extends Expr1

case class ArrayLiter(elems: List[Expr1]) extends Literal

sealed trait SemanticType {
  type T // the type with which a literal of this semanticType is represented
}
case object SemanticInt extends SemanticType {
  type T = Int
}

case class SemanticArray[U <: SemanticType](dim: Int) extends SemanticType {
  type T = List[U]
}

sealed trait Expr2[+T]
class Liter[T <: SemanticType](val ty: T, val value: ty.T) extends Expr2[T]

def typecheckArrayLiter(
    a: ArrayLiter
): Liter[SemanticArray[? <: SemanticType]] = { // bit of a hack here, usually we would just make SemanticArray type parameter covariant, but here we would get an error: "covariant type U occurs in invariant position in type  = List[U] of type T"
  val x: List[SemanticInt.type] = List() // removed Expr2 from type declaration
  Liter(SemanticArray[SemanticInt.type](0), x) // added a missing argument
}

[som-snytt]

I would expect "cannot reproduce" to mean the reported symptom can't be reproduced in a lab setting, rather than "can't be reproduced because the bug is already fixed." (I happened to arrive here because "cannot reproduce" is also a challenge.)

[nicolasstucki]

@som-snytt you are right "cannot reproduce" should be used when we can't be reproduced in a lab setting.

From what I understand, we could reproduce the issue on 3.2.2. Is that right @jchyb?<\del>

It does fail as reported in 3.2.2.