Compiler crash using partial function that returns F[_] inside value class

[grzegorz-bielski]

Compiler version

3.3.1

Minimized code

//> using scala 3.3.1

trait Arb[F[_]]:
  def pure[A](x: A): F[A]

implicit class PfOps(self: Int) extends AnyVal {
    def pf[F[_]](using m: Arb[F]): PartialFunction[Int, F[Int]] = m.pure(_)
}

The example starts working again if either AnyVal or F[_] is removed, or PartialFunction is changed to Function1 or implicit class is converted to Scala 3 extension method, but it also fails without implicit keyword.

Output (click arrow to expand)

Compiling project (Scala 3.3.1, JVM)
Error: Error while emitting /Users/grzegorzbielski/projects/my/delete-me/Main.scala
assertion failed: Unexpected alias type: type F
Error compiling project (Scala 3.3.1, JVM)
Compilation failed

Full stack trace:

java.lang.AssertionError: assertion failed: Unexpected alias type: type F
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.transform.GenericSignatures$.jsig$1(GenericSignatures.scala:261)
        at dotty.tools.dotc.transform.GenericSignatures$.boxedSig$1(GenericSignatures.scala:68)
        at dotty.tools.dotc.transform.GenericSignatures$.argSig$1(GenericSignatures.scala:195)
        at dotty.tools.dotc.transform.GenericSignatures$.classSig$1$$anonfun$1(GenericSignatures.scala:229)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:333)
        at dotty.tools.dotc.transform.GenericSignatures$.classSig$1(GenericSignatures.scala:229)
        at dotty.tools.dotc.transform.GenericSignatures$.jsig$1(GenericSignatures.scala:288)
        at dotty.tools.dotc.transform.GenericSignatures$.boxedSig$1(GenericSignatures.scala:68)
        at dotty.tools.dotc.transform.GenericSignatures$.superSig$1$$anonfun$1(GenericSignatures.scala:65)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:333)
        at dotty.tools.dotc.transform.GenericSignatures$.superSig$1(GenericSignatures.scala:65)
        at dotty.tools.dotc.transform.GenericSignatures$.jsig$1(GenericSignatures.scala:348)
        at dotty.tools.dotc.transform.GenericSignatures$.javaSig0(GenericSignatures.scala:365)
        at dotty.tools.dotc.transform.GenericSignatures$.javaSig(GenericSignatures.scala:40)
        at dotty.tools.backend.jvm.BCodeHelpers.dotty$tools$backend$jvm$BCodeHelpers$$getGenericSignatureHelper(BCodeHelpers.scala:773)
        at dotty.tools.backend.jvm.BCodeHelpers$BCJGenSigGen.getGenericSignature(BCodeHelpers.scala:400)
        at dotty.tools.backend.jvm.BCodeHelpers$BCJGenSigGen.getGenericSignature$(BCodeHelpers.scala:381)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.getGenericSignature(BCodeSkelBuilder.scala:74)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.initJClass(BCodeSkelBuilder.scala:273)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:123)
        at dotty.tools.backend.jvm.CodeGen.genClass(CodeGen.scala:138)
        at dotty.tools.backend.jvm.CodeGen.genClassDef$1(CodeGen.scala:55)
        at dotty.tools.backend.jvm.CodeGen.genClassDefs$1(CodeGen.scala:101)
        at dotty.tools.backend.jvm.CodeGen.genClassDefs$1$$anonfun$1(CodeGen.scala:99)
        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$1(CodeGen.scala:99)
        at dotty.tools.backend.jvm.CodeGen.genUnit(CodeGen.scala:104)
        at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:76)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:327)
        at scala.collection.immutable.List.map(List.scala:246)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:331)
        at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:87)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1321)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:262)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:270)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:279)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:67)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:279)
        at dotty.tools.dotc.Run.compileSources(Run.scala:194)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        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 /Users/grzegorzbielski/projects/my/dotty-error-delete-me/Main.scala
assertion failed: Unexpected alias type: type F
1 error found
Compilation failed

[sjrd]

Very slightly minimized (no implicit), and with different names for F in Arb (Fx) than in def pf (Fy). The crash is about Fy:

trait Arb[Fx[_]]:
  def pure[A](x: A): Fx[A]

class PfOps(self: Int) extends AnyVal {
  def pf[Fy[_]](m: Arb[Fy]): PartialFunction[Int, Fy[Int]] = {
    case x => m.pure(x)
  }
}

This happens during the back-end, when generating Java generic signatures. I suspect it's trying to compute the type of a synthetic method that lacks proper erasure, for some reason.