Pickling crash compiling `perspective` from community build

[griggt]

Since #13650, compiling perspective results in a crash (it was disabled in the community build by 5793f90)

A self-contained version of the offending code is below.

/cc @Katrix

Compiler version

3.1.1-RC1-bin-SNAPSHOT-git-8c3e7a2

Minimized code

type Const[A] = [_] =>> A
type FunctionK[A[_], B[_]] = [Z] => A[Z] => B[Z]

type ~>:[A[_], B[_]] = FunctionK[A, B]
type ~>#:[F[_], R] = F ~>: Const[R]
type #~>#:[T, R] = Const[T] ~>: Const[R]

object FunctionK:
  def liftConst[A, B](f: A => B): A #~>#: B = [Z] => (a: A) => f(a)

trait FoldableK[F[_[_], _]]:
  extension[A[_], C](fa: F[A, C]) 
    def foldMapK[B](f: A ~>#: B): B

  extension[A, C](fa: F[Const[A], C])
    def toListK: List[A] = fa.foldMapK(FunctionK.liftConst(List(_: A)))

Output (click arrow to expand)

error when pickling type Z
error when pickling type Const[List[A]][Z]
error when pickling tree Const[List[A]][Z]
error when pickling tree FoldableK.this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]]
error when pickling tree FoldableK.this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]](
  FunctionK.liftConst[A, List[A]](
    {
      def $anonfun(_$11: A): List[A] = List.apply[A]([_$11:A : A]*)
      closure($anonfun)
    }
  )
)
error when pickling tree extension [A >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[Const[A], C]) def toListK: List[A] = 
  this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]](
    FunctionK.liftConst[A, List[A]](
      {
        def $anonfun(_$11: A): List[A] = List.apply[A]([_$11:A : A]*)
        closure($anonfun)
      }
    )
  )
error when pickling tree [F[_[_ >: Nothing <: Any] >: Nothing <: Any, _ >: Nothing <: Any] >: Nothing <: Any]() extends Object {
  F[_$7[_$8],_$9]
  extension [A[_ >: Nothing <: Any] >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[A, C]) def foldMapK[
    B >: Nothing <: Any
  ](f: ~>#:[A, B]): B
  extension [A >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[Const[A], C]) def toListK: List[A] = 
    this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]](
      FunctionK.liftConst[A, List[A]](
        {
          def $anonfun(_$11: A): List[A] = List.apply[A]([_$11:A : A]*)
          closure($anonfun)
        }
      )
    )
}
error when pickling tree @SourceFile("FoldableK.scala") trait FoldableK[F[_[_ >: Nothing <: Any] >: Nothing <: Any, _ >: Nothing <: Any] >: Nothing <: Any
  ]
() extends Object {
  F[_$7[_$8],_$9]
  extension [A[_ >: Nothing <: Any] >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[A, C]) def foldMapK[
    B >: Nothing <: Any
  ](f: ~>#:[A, B]): B
  extension [A >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[Const[A], C]) def toListK: List[A] = 
    this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]](
      FunctionK.liftConst[A, List[A]](
        {
          def $anonfun(_$11: A): List[A] = List.apply[A]([_$11:A : A]*)
          closure($anonfun)
        }
      )
    )
}
error when pickling tree package <empty> {
  @SourceFile("FoldableK.scala") trait FoldableK[
    F[_[_ >: Nothing <: Any] >: Nothing <: Any, _ >: Nothing <: Any] >: Nothing <: Any
  ]() extends Object {
    F[_$7[_$8],_$9]
    extension [A[_ >: Nothing <: Any] >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[A, C]) def foldMapK[
      B >: Nothing <: Any
    ](f: ~>#:[A, B]): B
    extension [A >: Nothing <: Any, C >: Nothing <: Any](fa: FoldableK.this.F[Const[A], C]) def toListK: List[A] = 
      this.foldMapK[([_] =>> A), C](fa)[Const[List[A]][Z]](
        FunctionK.liftConst[A, List[A]](
          {
            def $anonfun(_$11: A): List[A] = List.apply[A]([_$11:A : A]*)
            closure($anonfun)
          }
        )
      )
  }
}
exception occurred while compiling FoldableK.scala
java.lang.AssertionError: assertion failed: orphan parameter reference: TypeParamRef(Z) while compiling FoldableK.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: orphan parameter reference: TypeParamRef(Z)
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:289)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$11$$anonfun$1(TreePickler.scala:181)
	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.tasty.TreePickler.pickleNewType$$anonfun$2(TreePickler.scala:181)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:181)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:604)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTpt(TreePickler.scala:316)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$39$$anonfun$1(TreePickler.scala:435)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$4(TreePickler.scala:435)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:436)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$3(TreePickler.scala:427)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:429)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTreeUnlessEmpty(TreePickler.scala:319)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$1(TreePickler.scala:335)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef(TreePickler.scala:337)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:560)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats$$anonfun$2(TreePickler.scala:360)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats(TreePickler.scala:360)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$26(TreePickler.scala:586)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:587)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$1(TreePickler.scala:332)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef(TreePickler.scala:337)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:562)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats$$anonfun$2(TreePickler.scala:360)
	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.tasty.TreePickler.pickleStats(TreePickler.scala:360)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$29(TreePickler.scala:602)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:602)
	at dotty.tools.dotc.core.tasty.TreePickler.pickle$$anonfun$1(TreePickler.scala:773)
	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.tasty.TreePickler.pickle(TreePickler.scala:773)
	at dotty.tools.dotc.transform.Pickler.run$$anonfun$3$$anonfun$2(Pickler.scala:69)
	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.Pickler.run$$anonfun$1(Pickler.scala:106)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.Pickler.run(Pickler.scala:106)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:308)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:309)
	at dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:111)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:261)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:272)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:280)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:289)
	at dotty.tools.dotc.Run.compileSources(Run.scala:222)
	at dotty.tools.dotc.Run.compile(Run.scala:206)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
	at dotty.tools.dotc.Driver.process(Driver.scala:199)
	at dotty.tools.dotc.Driver.process(Driver.scala:167)
	at dotty.tools.dotc.Driver.process(Driver.scala:179)
	at dotty.tools.dotc.Driver.main(Driver.scala:209)
	at dotty.tools.dotc.Main.main(Main.scala)

[odersky]

I minimized further to

type Const[A] = [_] =>> A
type FunctionK[A[_], B[_]] = [Z] => A[Z] => B[Z]

type #~>#:[T, R] = FunctionK[Const[T], Const[R]]

object FunctionK:
  def liftConst[A, B](f: A => B): /*FunctionK[Const[A], Const[B]]*/ A #~>#: B =
    [Z1] => (a: A) => f(a)

trait FoldableK[F[_[_], _]]:

  def foldMapK1[A[_], C, B](fa: F[A, C])(f: FunctionK[A, Const[B]]): B

  def toListK[A, C](fa: F[Const[A], C]): List[A] =
    foldMapK1(fa)(FunctionK.liftConst(List(_: A)))

Interestingly, if I expand the alias A #~>#: B in the definition of liftConst to FunctionK[Const[A], Const[B]] everything compiles.

What happens is:

1. We compare at some stage a polymorphic function with the poly function type [Z] => A[Z] => B[Z].
2. This causes a constraint B >: Const[List[A]][Z] to be added.
3. The comparison returns true with this added constraint.
4. But the Z binding goes out of scope after the comparison.

Hence the orphan parameter. It looks like another instance of the problem where we have to avoid certain parameters in a constraint. @smarter, do we have a solution for that yet?

[smarter]

Thanks for the minimization, I have wip branch where I'm working on this sort of issues so I'll keep this in mind.

[odersky]

It turns out this had a more obvious fix: Just treat PolyTypes like HKTypeLambdas.