Kind error in simple inheritance

[szeiger]

We're running into an error compiling collection-strawman on Dotty 0.4.0-RC1. Here's the branch with the reproduction: https://github.com/scala/collection-strawman/compare/wip/dotty-kind-error

Steps to reproduce:

# Cleaning in sbt is bugged in our build; do it manually to make sure you rebuild everything
$ rm -r collections/.jvm/target/

$ sbt ++0.4.0-RC1 collectionsJVM/test:compile
[...]
[info] Compiling 86 Scala sources and 5 Java sources to /Users/szeiger/code/scala/collection-strawman/collections/.jvm/target/scala-0.4/classes...
[...]
[warn] 12 warnings found
[info] Compiling 1 Scala source to /Users/szeiger/code/scala/collection-strawman/collections/.jvm/target/scala-0.4/test-classes...
[error]
[error] Type argument strawman.collection.immutable.IndexedSeq has not the same kind as its bound  <: [_$63] => scala.Any
[error] one error found
[error] (collectionsJVM/test:compileIncremental) Compilation failed

The error message does not give any position. It is caused by refining the type of ArrayOps.withFilter:

  override def withFilter(p: A => Boolean): ArrayOps.WithFilter[A] = new ArrayOps.WithFilter[A](p, this)

Without the type annotation here (or by explicitly annotating it with the original type collection.WithFilter[A, immutable.IndexedSeq]) the test class compiles. I have been unable to come up with any workaround that would allow us to refine the type (as required to get some extra methods).

The IndexedSeq type argument mentioned in the error message comes from the definition of ArrayOps.WithFilter:

class WithFilter[A](p: A => Boolean, ao: ArrayOps[A]) extends collection.WithFilter[A, immutable.IndexedSeq]

This error is a blocker for scala/collection-strawman#278

[allanrenucci]

I was able to reproduce:

In collection.scala:

package collection

abstract class WithFilter[+A, +CC[_]]

trait IndexedSeq[+A] extends Any with IndexedSeqOps[A, IndexedSeq, IndexedSeq[A]]

trait IndexedSeqOps[+A, +CC[X] <: IndexedSeq[X], +C] extends Any {
  def withFilter(p: A => Boolean): WithFilter[A, CC] = ???
}

package immutable {
  trait IndexedSeq[+A] extends collection.IndexedSeq[A] with collection.IndexedSeqOps[A, IndexedSeq, IndexedSeq[A]]
}

object ArrayOps {
  abstract class WithFilter[A] extends collection.WithFilter[A, immutable.IndexedSeq]
}

class ArrayOps[A](val xs: Array[A]) extends AnyVal with IndexedSeqOps[A, immutable.IndexedSeq, Array[A]] {
  override def withFilter(p: A => Boolean): ArrayOps.WithFilter[A] = ???
}

In Test.scala:

package collection

class Test {
  def test(xs: Array[Int]): Unit = {
    new ArrayOps(xs)
  }
}

dotty(master|…) ❯ ./bin/dotc collection.scala; ./bin/dotc Test.scala

Type argument collection.immutable.IndexedSeq has not the same kind as its bound  <: [_$1] => Any

one error found

[allanrenucci]

@szeiger I tested the fix on wip/dotty-kind-error and confirms that the tests now compile. The fix will be in Dotty 0.5.0-RC1 which will be released on Wednesday