Scala 3 Ambiguous Implicits in UnorderedFoldable

[bpholt]

Using cats 2.9.0 and Scala 3.2.1, this code results in a migration warning (or an error in Scastie)

import cats.syntax.all._

val list: List[(String, String)] = List.empty

list
  .nested
  .map(_.length)
  .value

I saw this locally:

Migration Warning:
Ambiguous implicits method catsTraverseForSeq in object UnorderedFoldable and method catsTraverseForList in object UnorderedFoldable seem to be used to implement a local failure in order to negate an implicit search. According to the new implicit resolution rules this is no longer possible; the search will fail with a global ambiguity error instead.

Consider using the scala.util.NotGiven class to implement similar functionality.

and see this error in Scastie (which is actually Scala 3.2.0, and isn't cross-building with sbt-typelevel, like I was locally)

Found: cats.data.Nested[F, G, A]
Required: ?{ map: ? }

where: A is a type variable with constraint >: String
F is a type variable with constraint >: List and <: [_] =>> Any
G is a type variable with constraint >: [T2] =>> (String, T2) and <: [_] =>> Any

Note that implicit conversions cannot be applied because they are ambiguous;
both method catsTraverseForList in object UnorderedFoldable and method catsTraverseForSeq in object UnorderedFoldable match type cats.Traverse[F]

[armanbilge]

If anyone wants to give it a try: I wonder if we can fix this by de-prioritizing the Seq instances in implicit search:

cats/core/src/main/scala/cats/UnorderedFoldable.scala

Lines 127 to 129 in f2c78d9

	implicit def catsTraverseForList: Traverse[List] = cats.instances.list.catsStdInstancesForList
	implicit def catsTraverseForSeq: Traverse[Seq] = cats.instances.seq.catsStdInstancesForSeq
	implicit def catsTraverseForVector: Traverse[Vector] = cats.instances.vector.catsStdInstancesForVector

[UlisesTorrella]

Hi, if no one is working on it yet I can give it a try

[armanbilge]

@UlisesTorrella yes please, thank you!

[UlisesTorrella]

I've implemented the de-prioritization of the seq instances in UnorderedFoldable like this:

trait LowPriorityImplicits {
  implicit def catsTraverseForSeq: Traverse[Seq] = cats.instances.seq.catsStdInstancesForSeq
}

object UnorderedFoldable
    extends ScalaVersionSpecificTraverseInstances
    with cats.instances.NTupleUnorderedFoldableInstances with LowPriorityImplicits{ ...

And though it solves this issue, it rises plenty warnings of the type the type test for cats.*.*[A] cannot be checked at runtime
I'll be checking other alternatives later, unless it's ok like this (I'm new to the repo)

[armanbilge]

@UlisesTorrella nice work! Would you mind putting up a PR for further discussion, and to give CI a crack at it? Thanks!