ambiguous contains_ syntax

[armanbilge]

Frustrating I can't make a small reproducer, but here it is in the wild.
https://github.com/gemini-hlsw/explore/runs/6928906317?check_suite_focus=true#step:7:56

The problem is that:

cats/core/src/main/scala/cats/syntax/unorderedFoldable.scala

Line 47 in 9839b74

def contains_(v: A)(implicit ev: Eq[A], F: UnorderedFoldable[F]): Boolean =

conflicts with

cats/core/src/main/scala/cats/syntax/foldable.scala

Line 73 in 9839b74

private[syntax] def contains_(v: A, eq: Eq[A], F: Foldable[F]): Boolean =

This is related to scala/bug#12578.

I thought we worked through it in #4183 since I linked it in that ticket, but seems there are still problems in some situations 🤔

[armanbilge]

Aha, here's a reproducer. The issue seems to be when using it as a lambda.

//> using scala "2.13.8"
//> using lib "org.typelevel::cats-core::2.8.0"

import cats.syntax.all._

object Test {
  def main(args: Array[String]): Unit = println(List("a").map(List("a").contains_))
}

[error] ./bug.scala:7:63: type mismatch;
[error]  found   : List[String]
[error]  required: ?{def contains_: ?}
[error] Note that implicit conversions are not applicable because they are ambiguous:
[error]  both method catsSyntaxFoldOps in trait FoldableSyntax of type [F[_], A](fa: F[A])(implicit evidence$2: cats.Foldable[F]): cats.syntax.FoldableOps[F,A]
[error]  and method catsSyntaxUnorderedFoldableOps in trait UnorderedFoldableSyntax of type [F[_], A](fa: F[A])(implicit evidence$1: cats.UnorderedFoldable[F]): cats.syntax.UnorderedFoldableOps[F,A]
[error]  are possible conversion functions from List[String] to ?{def contains_: ?}
[error]   def main(args: Array[String]): Unit = println(List("a").map(List("a").contains_))
[error]                                                               ^^^^^^^^^
[error] ./bug.scala:7:63: value contains_ is not a member of List[String]
[error] did you mean contains?
[error]   def main(args: Array[String]): Unit = println(List("a").map(List("a").contains_))
[error]                                                               ^^^^^^^^^^^^^^^^^^^

[armanbilge]

Rewriting as contains_(_) fixes it.

[joroKr21]

I guess it has to be fixes in the compiler - it works in Scala 3.
I didn't realise that it works when not treated as a lambda.

[armanbilge]

I didn't realise that it works when not treated as a lambda.

It does but only because we rewrote the argument list to be (_, _, _) instead of (_)(implicit _, _) precisely to try and avoid this issue.

[joroKr21]

Oh ok I see - it's a trick that changed the method signature but not binary compatibility.

[armanbilge]

Nobody else has complained, this is annoying to fix, and there's an easy workaround. I'm going to put this on the backlog.

[matthughes]

Came across this a bunch today when I upgraded to 2.8.0 on 2.13.8. Hope to upgrade to Scala 3 soon with this project but just FYI.

[armanbilge]

Darn, just when I thought I got away with it! Thanks for letting us know.

[mahe-ymc]

Came across this a bunch today when I upgraded to 2.8.0 on 2.13.8. Hope to upgrade to Scala 3 soon with this project but just FYI.

Similar here. Just fyi, workaround is easy enough.

[armanbilge]

I opened scala/bug#12666 with a minimizer of this issue. Also, I might have an idea to solve it.