AnyRef expected type for ArraySeq#to(ArraySeq) gives error type mismatch

[bjornregnell]

reproduction steps

using Scala 2.13,

Welcome to Scala 2.13.3 (OpenJDK 64-Bit Server VM, Java 11.0.8).
Type in expressions for evaluation. Or try :help.

scala> import collection.mutable.ArraySeq
import collection.mutable.ArraySeq

scala> val as = ArraySeq(1,2,3)
val as: scala.collection.mutable.ArraySeq[Int] = ArraySeq(1, 2, 3)

scala> as eq as.to(ArraySeq)
                   ^
       error: type mismatch;
        found   : scala.collection.mutable.ArraySeq.type
        required: scala.collection.Factory[Int,AnyRef]

problem

It works on e.g.

scala> as.to(ArraySeq)
val res0: scala.collection.mutable.ArraySeq[Int] = ArraySeq(1, 2, 3)

so it is a surprise.

[bjornregnell]

But there is a workaround if importing some stuff and calling it, according to https://gitter.im/scala/contributors?at=5f871a0824a20801a8e83333 (credit Tom Grigg)

[griggt]

The observation was that explicitly calling the implicit toFactory method avoids the error. It appears as though the implicit is not selected in the case where the eq is in the same expression:

Welcome to Scala 2.13.3 (OpenJDK 64-Bit Server VM, Java 11.0.6).
Type in expressions for evaluation. Or try :help.

scala> import collection.mutable.ArraySeq
import collection.mutable.ArraySeq

scala> val a = ArraySeq(1, 2, 3)
val a: scala.collection.mutable.ArraySeq[Int] = ArraySeq(1, 2, 3)

scala> val b = a.to(ArraySeq)
val b: scala.collection.mutable.ArraySeq[Int] = ArraySeq(1, 2, 3)

scala> a eq b
val res0: Boolean = false

scala> a == a.to(ArraySeq)
val res1: Boolean = true

scala> a eq a.to(ArraySeq)
                 ^
       error: type mismatch;
        found   : scala.collection.mutable.ArraySeq.type
        required: scala.collection.Factory[Int,AnyRef]

scala> import scala.collection.EvidenceIterableFactory.toFactory
import scala.collection.EvidenceIterableFactory.toFactory

scala> a eq a.to(toFactory(ArraySeq))
val res3: Boolean = false

[som-snytt]

IOW

scala> val xs: AnyRef = ArraySeq(17).to(ArraySeq)
                                        ^
       error: type mismatch;
        found   : scala.collection.mutable.ArraySeq.type
        required: scala.collection.Factory[Int,AnyRef]

scala> val xs: ArraySeq[_] = ArraySeq(17).to(ArraySeq)
val xs: scala.collection.mutable.ArraySeq[_] = ArraySeq(17)

[bjornregnell]

Aha. I guess the title is wrong then as this does not seem to be depending on the eq test? @som-snytt Could you, if appropriate, help me with a better title?

[bjornregnell]

Thanks! Wonderful title 👍

[SethTisue]

Tentatively labeled library:collections unless someone can establish that it's actually a compiler issue.

It's the same in Dotty (0.27.0-RC1).

[lrytz]

In List(1).to(List): AnyRef, the implicit search looks for

[search #1] start `collection.immutable.List.type`, searching for adaptation to pt=collection.immutable.List.type => scala.collection.Factory[Int,AnyRef]

Factory[Int, AnyRef] is a valid type, but

implicit def toFactory[A, CC[_]](factory: IterableFactory[CC]): Factory[A, CC[A]]

doesn't fit the bill, it requires a type constructor for CC, so toFactory[Int, AnyRef] doesn't work.

It works for List(1).to(List): Any, because Any is treated kind-polymorphic (toFactory[Int, Any](List) is allowed).

Maybe we need to add another implicit

implicit def toAnyRefFactory[A, C[_] <: AnyRef](factory: collection.IterableFactory[C]): collection.Factory[A, AnyRef] =
    collection.IterableFactory.toFactory(factory)

[NthPortal]

where would/could we add that implicit?

[lrytz]

I don't know, maybe in a LowPriorityImplicits parent of IterableFactory to minimize the risk of turning currently working implicit resolutions ambiguous.

Anyway there's not much we can do right now due to binary compatibility. But adding implicits needs to be done carefully. For example in 2.12, there's Predef.fallbackStringCanBuildFrom[T]: CanBuildFrom[String, T, immutable.IndexedSeq[T]] which is used much more often than anticipated:

scala> Iterable(1, 2).to[Seq] // print
   scala.`package`.Iterable.apply[Int](1, 2).to[scala.`package`.Seq](scala.Predef.fallbackStringCanBuildFrom[Int]) // : Seq[Int]

scala> Iterable(1, 2).to[Seq] // print
res0: Seq[Int] = Vector(1, 2)

[som-snytt]

Ultimately everything is a string?

My other earnest PR for pretty-printing has it backwards, instances don't "wind up" as strings, they start out that way.