Anonymous classes methods becoming private when compiled.

[scabug]

When a type of an anonymous class is less specific that the definition of the class (AnyRef for example) the anonymous class methods become private. This happens with val, def and var definition inside the class.

  type T = {val x: String; def y: String; var z: String}

  // This one generates to compile the anonymous class properly.
  // It marks all methods and field accessors as private
  // If the return type is T then it compiles correctly
  def get(): AnyRef = new { val x = "x"; def y = "y"; var z = "z"}

  val obj = get()
  // Prints: Failure(java.lang.NoSuchMethodException: A$$anon$2.x())
  println(Try(obj.asInstanceOf[T].x))

  // Prints: Failure(java.lang.NoSuchMethodException: A$$anon$2.y())
  println(Try(obj.asInstanceOf[T].y))

  // Prints: Failure(java.lang.NoSuchMethodException: A$$anon$2.z())
  println(Try(obj.asInstanceOf[T].z))
  // Failure(java.lang.NoSuchMethodException: A$$anon$1.z_$eq(java.lang.String))
  println(Try(obj.asInstanceOf[T].z = "zz"))
  // Prints: Failure(java.lang.NoSuchMethodException: A$$anon$2.z())
  println(Try(obj.asInstanceOf[T].z))

  println
  println("Fields:")
  // Lists x and z fields as expected
  obj.getClass.getDeclaredFields.foreach(x => println(s"  $x"))
  println("Methods:")
  // All methods are private but should be public
  obj.getClass.getDeclaredMethods.foreach(x => println(s"  $x"))

It looks like a code pattern that should be avoided, but in Scala.js this is used to create javascript objects. scala-js/scala-js#1899

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9487?orig=1
Reporter: Nicolas Stucki (nicolas.stucki)
Affected Versions: 2.11.7, 2.12.0-M3

[scabug]

@sjrd said:
Note that the methods and fields are already private directly after typer, as can be seen with -Xprint:typer. So the Component should probably be the typechecker instead of the backend.

[scabug]

@retronym said:
Here's the backstory: https://github.com/scala/scala/blob/v2.11.7/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L2327-L2344

[scabug]

@sjrd said:
Hum, that looks related indeed, and the comment suggests this is Hard (tm). However, IIUC, the linked backstory explains why the type of the Block needs to hide away some members. But I don't think that explains why the actual class itself (its implementation) needs to suddenly turn public members into private ones. It seems to me that there might be a relatively easy fix to that (compared to fixing the type issue).

It is also worth mentioning that if the anonymous class does not have any expected type at all, then the members are kept public. For example rewriting get() above as:

def get(): AnyRef = {
  val o = new { ... }
  o
}

allows all members to be public.

[som-snytt]

This came up recently because the private members can be reported unused in Scala 2.

Scala 3 has to do extra work to decide if a member is "effectively private".

scala/scala3#22729

Is it OK to close this as out of scope a decade later? Scala 3 made a different choice, but Scala 2 is unlikely to change.

[sjrd]

Yes, let's close.