ambiguous implicit object arguments for contravariant trait

[fdietze]

Compiler version

3.1.1-RC1, ScalaJS

Minimized code

import scala.scalajs.js

trait Render[-T] { def render(value: T): String }

implicit object StringRender extends Render[String] {
  def render(value: String): String = value
}

implicit object UndefOrRender extends Render[js.UndefOr[String]] {
  def render(value: js.UndefOr[String]): String = value.getOrElse("empty")
}

implicitly[Render[String]]

https://scastie.scala-lang.org/mVuFoqtXTbi89svihWkgeg

Output

ambiguous implicit arguments:
both object StringRender in object Playground
and object UndefOrRender in object Playground
match type Playground.Render[String] of parameter e of method implicitly in object Predef

Expectation

Should compile like in Scala 2.13

Notes

It works when making the trait Render invariant.
It also works when using implicit val instead of implicit object:

import scala.scalajs.js

trait Render[-T] { def render(value: T): String }

implicit object StringRender extends Render[String] {
  def render(value: String): String = value
}

implicit object UndefOrRender extends Render[js.UndefOr[String]] {
  def render(value: js.UndefOr[String]): String = value.getOrElse("empty")
}

implicitly[Render[String]]

https://scastie.scala-lang.org/mVuFoqtXTbi89svihWkgeg

[sjrd]

This doesn't look related to Scala.js per se. It looks like a typer thing. I can reproduce the issue using Scala/JVM:
https://scastie.scala-lang.org/dENS4y3SReesiZGYKLrLDA

trait Render[-T] { def render(value: T): String }

implicit object StringRender extends Render[String]:
  def render(value: String): String = value
end StringRender

implicit object UndefOrRender extends Render[scala.|[String, Unit]] {
  def render(value: scala.|[String, Unit]): String =
    if value == () then "empty" else value.asInstanceOf[String]
}

implicitly[Render[String]]
// ambiguous implicit arguments: both object StringRender in object Playground and object UndefOrRender in object Playground match type Playground.Render[String] of parameter e of method implicitly in object Predef

[odersky]

Seems nobody wants to take this on. I don't know enough about scala.I to figure out what goes on here. My guess is that it's the different handling of contravariant parameters, which would mean that Scala 3's behavior is as intended.

[smarter]

I think it's just that neither object type is more specific than the other, if you use implicit vals or givens then it works out since we end up comparing the types Render[String] to Render[String | Unit] and one is more specific than the other:

trait Render[-T] { def render(value: T): String }

given StringRender: Render[String] with
  def render(value: String): String = value

given UndefOrRender: Render[scala.|[String, Unit]] with
  def render(value: scala.|[String, Unit]): String =
    if value == () then "empty" else value.asInstanceOf[String]

implicitly[Render[String]]