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.

1 files changed, 9 insertions, 4 deletions

diff --git a/test/files/neg/valueclasses.scala b/test/files/neg/valueclasses.scala
index 7cac94ab11..06fde40a70 100644
--- a/test/files/neg/valueclasses.scala
+++ b/test/files/neg/valueclasses.scala
@@ -11,10 +11,10 @@ class Foo {
 
 class V1 extends AnyVal // fail
 
-class V2(private[test] val x: Int) extends AnyVal // fail
-class V3(protected[test] val x: Int) extends AnyVal // fail
-class V4(protected val x: Int) extends AnyVal // fail
-class V5(private val x: Int) extends AnyVal // fail
+class V2(private[test] val x: Int) extends AnyVal // okay, wasn't allowed in 2.10.x
+class V3(protected[test] val x: Int) extends AnyVal // okay, wasn't allowed in 2.10.x
+class V4(protected val x: Int) extends AnyVal // okay, wasn't allowed in 2.10.x
+class V5(private val x: Int) extends AnyVal // okay, wasn't allowed in 2.10.x
 
 class V6(val x: Int, val y: String) extends AnyVal // fail
 class V7(val x: Int, private[this] val y: String) extends AnyVal // fail
@@ -29,3 +29,8 @@ class V11[T](val x: List[T]) extends AnyVal // ok
 class V12[@specialized T, U](val x: (T, U)) extends AnyVal // fail
 
 class V13(x: Int) extends AnyVal // fail
+
+class V14(private[this] val x: Int) extends AnyVal // fail
+class V15(protected[this] val x: Int) extends AnyVal // fail
+
+class V16()(val a: Any) extends AnyVal // fail, was allowed 2.10.x