Not actionable error about unaccurate Scala 2 existential type

[cchantep]

Compiler version

3.1.0

Minimized code

// compile with 2.13.7
import language.existentials

abstract class Box[T]

object Box {
  def id(b: Box[Box[x]] forSome { type x }) = b
}

// compile with 3.1.0
@main def test =
  val b = Box.id(??? : Box[Box[Int]])

Output

An existential type that came from a Scala-2 classfile cannot be
mapped accurately to to a Scala-3 equivalent.
original type    : Box[Box[x]] forSome x
reduces to       : Box[Box[x]]
type used instead: Box[Box[Any]]
This choice can cause follow-on type errors or hide type errors.
Proceed at own risk.

longer explanation available when compiling with `-explain`
An existential type that came from a Scala-2 classfile cannot be
mapped accurately to to a Scala-3 equivalent.
original type    : Box[Box[x]] forSome x
reduces to       : Box[Box[x]]
type used instead: Box[Box[Any]]
This choice can cause follow-on type errors or hide type errors.
Proceed at own risk.

longer explanation available when compiling with `-explain`

Expectation

Mention which classfile is causing the issue.

[bishabosha]

I added some minimised code

[ckipp01]

I think this is a duplicate of #13873 and is already fixed by #13879

[smarter]

Indeed, fixed in 3.1.2-RC1.