ListBuffer, Seq and IndexedSeq cannot be compiled together

[DarkDimius]

dotc ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala -Ycheck:all ./scala-scala/src/library/scala/collection/immutable/Seq.scala ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala

checking ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala after phase frontend
checking ./scala-scala/src/library/scala/collection/immutable/Seq.scala after phase frontend
checking ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala after phase frontend
checking ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala after phase posttyper
checking ./scala-scala/src/library/scala/collection/immutable/Seq.scala after phase posttyper
checking ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala after phase posttyper
checking ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala after phase pickler
checking ./scala-scala/src/library/scala/collection/immutable/Seq.scala after phase pickler
checking ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala after phase pickler
checking ./scala-scala/src/library/scala/collection/mutable/ListBuffer.scala after phase TreeTransform:{firstTransform, checkReentrant}
checking ./scala-scala/src/library/scala/collection/immutable/Seq.scala after phase TreeTransform:{firstTransform, checkReentrant}
checking ./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala after phase TreeTransform:{firstTransform, checkReentrant}
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$GenMapFactory$$CC; assuming they are invariant
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$MapFactory$$CC; assuming they are invariant
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$GenSetFactory$$CC; assuming they are invariant
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$SetFactory$$CC; assuming they are invariant
./scala-scala/src/library/scala/collection/mutable/IndexedSeq.scala:26: error: overriding method companion in trait IndexedSeq of type =>
  scala.collection.generic.GenericCompanion[[+hk$0] =>
    scala.collection.IndexedSeq[hk$0]
  ];
 method companion of type =>
  scala.collection.generic.GenericCompanion[[hk$0] =>
    scala.collection.mutable.IndexedSeq[hk$0]
  ] has incompatible type
  override def companion: GenericCompanion[IndexedSeq]  = IndexedSeq
                 ^
four warnings found
one error found
there were 2 deprecation warning(s); re-run with -deprecation for details

[odersky]

That's a tricky issue, but it could be that Dotty is right in rejecting this override.

GenericCompanion is covariant, so we'd need that collection.mutable.IndexedSeq (as a hk type) is a subtype of collection.IndexedSeq. But collection.IndexedSeq is covariant wherewas collection.mutable.IndexedSeq is not. This seems to violate the LSP, and could also lead to an unsoundness problem.

@adriaanm @retronym Do you have an opinion on this?

[retronym]

Will need to take a deeper look at this, but it seems closely related to scala/scala#3184 (maybe that didn't go far enough).

[retronym]

I'm trying to make a small test case to show this issue that we can discuss. I don't think I've got there, the following compiles both in recent dotc and with scalac.

import annotation.unchecked._
import language._

class Inv[A]
class Cov[+A] extends Inv[A @uncheckedVariance]

trait M[+F[A]]

class Test {
  def f(m: M[Cov]) : M[Inv] = m
}

[smarter]

@retronym: I think this is what you were trying to get to:

import annotation.unchecked._
import language._

class Cov[+A]
class Inv[A] extends Cov[A]

trait M[+F[A]]

class Test {
  def f(m: M[Inv]) : M[Cov] = m
}

[odersky]

I think it is not a problem, after all. The point is that the definition of generic companion is as follows:

GenericCompanion[+CC[X] <: GenTraversable[X]]

That is, CC is not expected to be covariant. This means that we cannot make use of the variance of
any type argument anyway, or, otherwise put, that variance of type arguments does not matter.