SI-7859 Value classes may wrap a non-public member

We allow value class constructors to be non-public, so to be regular,
we should also allow the same for the param accessor.

This commit uses the 'makeNotPrivate' machinery to ensure that
the backend can generate the requisite unboxing calls.

This commit:

  - refactors the code that enforced the restrictions, improving
    a few error messages and positions. The remaining restrictions
    needed some rewording in light of this change.
  - allows value classes to have non-public, val parameters.
    private[this] / protected[this] are still disallowed as value
    classes don't have a concept of `this`, and because trying to
    accomdate then would complicate the implementation.

    This means that `class C(x: Int) extends AnyVal` is not allowed,
    the user still must write `private val x: Int` or `val x: Int`.
  - Outlaw `class C()()(val x: Int) extends AnyVal` to curtail any
    bugs that might lurk in such a formulation.

The tests:

  - Show that the privacy is respected in the typer phase, under
    joint and separate compilation. We don't want a repeat performance
    of SI-6601.
  - Show that code that needs compiler-generated unboxes works under
    both compilation scenarios
  - Checks that the remaining restrictions are enforced and well
    communicated.

2 files changed, 14 insertions, 0 deletions

diff --git a/test/files/neg/t7859/A_1.scala b/test/files/neg/t7859/A_1.scala
new file mode 100644
index 0000000000..e5b32d1c96
--- /dev/null
+++ b/test/files/neg/t7859/A_1.scala
@@ -0,0 +1,5 @@
+package p1 {
+  class A(private[p1] val x: Any) extends AnyVal
+}
+class B(private val x: Any) extends AnyVal
+
diff --git a/test/files/neg/t7859/B_2.scala b/test/files/neg/t7859/B_2.scala
new file mode 100644
index 0000000000..2e0556bc7b
--- /dev/null
+++ b/test/files/neg/t7859/B_2.scala
@@ -0,0 +1,9 @@
+class C(private val x: Any) extends AnyVal
+
+// Checking that makeNotPrivate(paramAccessor) doesn't make this visible during typer.
+// The output is identical with/without `extends AnyVal`.
+object Test {
+  new p1.A(x).x
+  new B(x).x
+  new C(x).x
+}