package-private methods ambiguate implicit conversions

[armanbilge]

Reproduction steps

Scala version: (Is the bug only in Scala 3.x? If so, report it at https://github.com/lampepfl/dotty/issues/new/choose instead.)

//> using scala "2.13.8"

package foo {
  package object syntax {
    implicit class BarOps1(a: Any) {
      def bar(x: Int): String = ???
    }

    implicit class BarOps2(a: Any) {
      private[syntax] def bar(x: Int): String = ???
    }
  }
}

import foo.syntax._

object Main {
  def main(args: Array[String]): Unit = {
    val a = new Object
    a.bar(42)
  }
}

Problem

[error] ./bug.scala:20:5: type mismatch;
[error]  found   : a.type (with underlying type Object)
[error]  required: ?{def bar(x$1: ? >: Int(42)): ?}
[error] Note that implicit conversions are not applicable because they are ambiguous:
[error]  both method BarOps1 in package syntax of type (a: Any): foo.syntax.package.BarOps1
[error]  and method BarOps2 in package syntax of type (a: Any): foo.syntax.package.BarOps2
[error]  are possible conversion functions from a.type to ?{def bar(x$1: ? >: Int(42)): ?}
[error]     a.bar(42)
[error]     ^
[error] ./bug.scala:20:5: value bar is not a member of Object
[error]     a.bar(42)
[error]     ^^^^^

It's surprising that the conversion is ambiguous, since BarOps2 should not have publicly visible methods. Note also that changing to private will solve the problem.

Credit to @BalmungSan for identifying the issue while working on typelevel/cats#4183.

[joroKr21]

I guess related to #6794

[armanbilge]

Ah, and is a dupe of #11787.

[joroKr21]

The place in Scalac is here: https://github.com/scala/scala/blob/2.13.x/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L1373-L1393

It might be possible to improve by passing the expected type (pt) to adaptToMemberWithArgs and using HasMethodMatching when it's a function.