Cons next leaks through `next$access$1`

[nicolasstucki]

Compiler version

3.3

Minimized code

The following code compiles but should not

def test(xs: ::[Int]): List[Int] =
  xs.next$access$1

Other manifestation

This method also apears in autocompletion

Expectation

This code should not compile.

[nicolasstucki]

Note that this accessor exists becase the next case parameter is private[scala]

final case class :: [+A](override val head: A, private[scala] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
  extends List[A] {

// scala 2 after typer
final case class ::[+A] extends List[A] with Product with Serializable {
  <caseaccessor> <paramaccessor> private[this] val head: A = _;
  override <stable> <caseaccessor> <accessor> <paramaccessor> def head: A = ::.this.head;
  <caseaccessor> <paramaccessor> private[this] var next: List[A] = _;
  <caseaccessor> <accessor> <paramaccessor> def next: List[A] = ::.this.next;
  <accessor> <paramaccessor> def next_=(x$1: List[A]): Unit = ::.this.next = x$1;
  def <init>(head: A, next: List[A]): Playground.::[A] = {
    ::.super.<init>();
    ()
  };
  ...
}

next$access$1 is created later in the superaccessors phase just before pickling.

[nicolasstucki]

This also leaks in mirrors (see tests/run/typeclass-derivation3.check).