Package-private nested case class in a library can be seen from other packages

[kubukoz]

Compiler version

3.3.1, 3.4.0-RC1-bin-20230911-1be790c-NIGHTLY

Minimized code

//> using scala "3.3.1"
//> using lib "org.typelevel::cats-effect:3.5.1"
import cats.effect.IOLocal
@main def demo = println(IOLocal.IOLocalImpl.apply(42))

Note: IOLocalImpl is defined as private[IOLocal] final class IOLocalImpl[A](default: A)

Output

cats.effect.IOLocal$IOLocalImpl@483bf400

Expectation

Compiler failure

Extra detail

I wasn't able to reproduce this without a library dependency.

[dwijnand]

package iolib:
  case class IO[A](value: A)

  sealed trait IOLocal[A]

  object IOLocal:
    def apply[A](default: A): IO[IOLocal[A]] = IO(new IOLocalImpl(default))

    private[IOLocal] final class IOLocalImpl[A](default: A) extends IOLocal[A]

package tests:
  import iolib.IOLocal
  def test = IOLocal.IOLocalImpl.apply(42)

[som-snytt]

Scala 2 also changes its messaging if the line is corrected:

package iolib {
  case class IO[A](value: A)

  sealed trait IOLocal[A]

  object IOLocal {
    def apply[A](default: A): IO[IOLocal[A]] = IO(new IOLocalImpl(default))

    private[IOLocal] final class IOLocalImpl[A](default: A) extends IOLocal[A]
  }
}

package tests {
  import iolib.IOLocal
  class C {
    def test = IOLocal.IOLocalImpl.apply(42)
    //def test = new IOLocal.IOLocalImpl(42)
  }
}

from

error: value IOLocalImpl is not a member of object iolib.IOLocal

to

error: class IOLocalImpl in object IOLocal cannot be accessed as a member of object iolib.IOLocal from class C in package tests

where Scala 3 misses the erroneous apply or constructor proxy.