Investigate why creating a `Monad[[X] => A => X]` instance causes ambiguous extension method errors

[mpilquist]

With function1.scala uncommented, expressions like List(1, 2).void result in errors like:

scala> List(1, 2).void
1 |List(1, 2).void
  |^^^^^^^^^^
  |Found:    List[Int]
  |Required: ?{ void: ? }
  |Note that implicit extension methods cannot be applied because they are ambiguous;
  |both method Function1Monad in package leopards and object ListMonad in package leopards provide an extension method `void` on List[Int]

[dscleaver]

Is this because of List’s apply method?

[dwijnand]

Yeah, I assume it's because List extends Seq extends PartialFunction extends Function1.

[mpilquist]

Aha! That makes sense, though I don't know how we should go about fixing here. I had hoped the more specific implied instance would have been selected.

[dscleaver]

Contextual clues help, but only a little:

scala> import implied leopards._

scala> val x : List[Unit] = List(1,2,3).void
val x: List[Unit] = List((), (), ())

scala> val x: Int => Unit = List(1,2,3).void
1 |val x: Int => Unit = List(1,2,3).void
| ^^^^^^^^^^^
|Found: List[Int]
|Required: ?{ void: ? }
|Note that implicit extension methods cannot be applied because they are ambiguous;
|both method Monad_Function1_instance in package leopards and object Monad_List_Traverse_List_instance in package leopards provide an extension method void on List[Int]

scala> val x: (Int => Unit) = (List(1,2,3): Int => Int).void
val x: Int => Unit = leopards.function1$package$Monad_Function1_instance$$Lambda$5817/671480273@789d585f

[mpilquist]

With 3.0.0-RC3:

[error] 7 |    assertEquals(List(1, 2) *> List(3, 4), List(3, 4, 3, 4))
[error]   |                 ^^^^^^^^^^
[error]   |Found:    List[Int]
[error]   |Required: ?{ *> : ? }
[error]   |Note that implicit extension methods cannot be applied because they are ambiguous;
[error]   |both given instance given_Monad_Function in package leopards and object given_Monad_List_Traverse_List in package leopards provide an extension method `*>` on List[Int]

[smarter]

@mpilquist There's a good chance this is the same issue as scala/scala3#12126

[smarter]

scala/scala3#12126 is now fixed, but this issue remains, after uncommenting src/main/scala/leopards/instances/function1.scala and setting scalaVersion to "3.0.3-RC1-bin-20210716-cc47c56-NIGHTLY" I get:

[root@test]> test:compile
[error] -- [E007] Type Mismatch Error: /home/smarter/opt/spotted-leopards/src/test/scala/leopards/FirstExample.scala:7:21 --------------------------------------------------------------------------------------------------
[error] 7 |    assertEquals(List(1, 2) *> List(3, 4), List(3, 4, 3, 4))
[error]   |                 ^^^^^^^^^^
[error]   |                 Found:    List[Int]
[error]   |                 Required: ?{ *> : ? }
[error]   |                 Note that implicit extension methods cannot be applied because they are ambiguous;
[error]   |                 both given instance given_Monad_Function in package leopards and object given_Monad_List_Traverse_List in package leopards provide an extension method `*>` on List[Int]
[error] one error found

So a minimization would still be useful here I think.

[smarter]

I have a fix I think: scala/scala3#13509

[smarter]

Fixed as of scalaVersion := "3.1.1-RC1-bin-20210916-dd6da26-NIGHTLY"