Implement experimental rule for unordering operation on Ordered collection

[MasseGuillaume]

Depends on: #121
Related to: scala/bug#11037

Summary:

map/flatMap behave differently on Ordered collection in 2.13 vs 2.12:

In Scala 2.12, if there is no Ordering when doing map or flatMap, it will convert the collection to the unordered one:

val bitset = collection.BitSet(1)

// with ordering
bitset.map(x => x) // collection.BitSet(1)

// without ordering
case class Unordered(v: Int)
bitset.map(x => Unordered(x)) // Set(Unordered(1))

In Scala 2.13, it's a compilation error:

bitset.map(x => Unordered(x)) // Set(Unordered(1))
          ^
error: No implicit Ordering defined for Unordered.

It's in the Experimental rules because we need the type of the collection (bitset in our example).

[julienrf]

I think we should not change 2.13’s syntax: we should produce code that uses unsorted.

[MasseGuillaume]

@julienrf we should fix: scala/bug#11037. Let's merge this PR and create a followup PR to change it back to unsorted.

[julienrf]

We should first fix the issue in scala/scala, and then open a PR here. I see no point in merging the current PR if we plan to undo it in a few days.

[MasseGuillaume]

Ok, I will fix scala/bug#11037 now.

[julienrf]

The issue has been fixed in Scala.

[MasseGuillaume]

@julienrf I'm not sure this was fixed. I only modified the .unsorted method to return the most precise collection. We still have the case where SortedMap.map(unOrdered) does not compile in 2.13.

Also, I forgot in this PR to handle for comprehensions.

[julienrf]

We still have the case where SortedMap.map(unOrdered) does not compile in 2.13

The cross-compatible way to achieve that is:

import scala.collection.compat._
SortedMap(1 -> 2).unsorted.map(toUnordered)

[MasseGuillaume]

@julienrf unsorted is not available in 2.12 I just have to rename unsortedSpecific to unsorted and remove unsortedSpecific from 2.13 in this PR. The rewrite rule in this PR is also valuable, you closed this too early.

[julienrf]

unsorted is not available in 2.12

It should be provided via import scala.collection.compat._.

Sorry for having closed this too early, I missed that we also needed something on the scala-collection-compat side.

[MasseGuillaume]

This is a both a blessing and a curse. It's good since it can allow us to catch bugs early, It's bad since it's makes things more fragile.

[MasseGuillaume]

Since .unsorted is only available in 2.13.0-pre-... I need a way to make the output compile with a complex build matrix. I tought it would make more sense to just remove the directory structure (output212, output212Plus, output213) and make this more fine-grained, per-file.

[MasseGuillaume]

@julienrf This PR is ready.