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, 23 insertions, 20 deletions

diff --git a/test/files/neg/valueclasses.check b/test/files/neg/valueclasses.check
index 3b82a8358c..35d38aae60 100644
--- a/test/files/neg/valueclasses.check
+++ b/test/files/neg/valueclasses.check
@@ -4,40 +4,43 @@ trait T extends AnyVal // fail
 valueclasses.scala:6: error: value class may not be a member of another class
   class Bar(x: Int) extends AnyVal // fail
         ^
+valueclasses.scala:6: error: value class parameter must be a val and not be private[this]
+  class Bar(x: Int) extends AnyVal // fail
+            ^
 valueclasses.scala:8: error: value class may not be a local class
     class Baz(x: Int) extends AnyVal // fail
           ^
-valueclasses.scala:12: error: value class needs to have exactly one public val parameter
+valueclasses.scala:8: error: value class parameter must be a val and not be private[this]
+    class Baz(x: Int) extends AnyVal // fail
+              ^
+valueclasses.scala:12: error: value class needs to have exactly one val parameter
 class V1 extends AnyVal // fail
       ^
-valueclasses.scala:14: error: value class needs to have a publicly accessible val parameter
-class V2(private[test] val x: Int) extends AnyVal // fail
-                           ^
-valueclasses.scala:15: error: value class needs to have a publicly accessible val parameter
-class V3(protected[test] val x: Int) extends AnyVal // fail
-                             ^
-valueclasses.scala:16: error: value class needs to have a publicly accessible val parameter
-class V4(protected val x: Int) extends AnyVal // fail
-                       ^
-valueclasses.scala:17: error: value class needs to have a publicly accessible val parameter
-class V5(private val x: Int) extends AnyVal // fail
-                     ^
-valueclasses.scala:19: error: value class needs to have exactly one public val parameter
+valueclasses.scala:19: error: value class needs to have exactly one val parameter
 class V6(val x: Int, val y: String) extends AnyVal // fail
       ^
-valueclasses.scala:20: error: field definition is not allowed in value class
+valueclasses.scala:20: error: value class needs to have exactly one val parameter
 class V7(val x: Int, private[this] val y: String) extends AnyVal // fail
-                                       ^
-valueclasses.scala:21: error: value class needs to have exactly one public val parameter
-class V8(var x: Int) extends AnyVal // fail
       ^
+valueclasses.scala:21: error: value class parameter must not be a var
+class V8(var x: Int) extends AnyVal // fail
+             ^
 valueclasses.scala:24: error: field definition is not allowed in value class
   val y = x    // fail
       ^
 valueclasses.scala:29: error: type parameter of value class may not be specialized
 class V12[@specialized T, U](val x: (T, U)) extends AnyVal // fail
                        ^
-valueclasses.scala:31: error: value class needs to have exactly one public val parameter
+valueclasses.scala:31: error: value class parameter must be a val and not be private[this]
 class V13(x: Int) extends AnyVal // fail
+          ^
+valueclasses.scala:33: error: value class parameter must be a val and not be private[this]
+class V14(private[this] val x: Int) extends AnyVal // fail
+                            ^
+valueclasses.scala:34: error: value class parameter must not be protected[this]
+class V15(protected[this] val x: Int) extends AnyVal // fail
+                              ^
+valueclasses.scala:36: error: value class needs to have exactly one val parameter
+class V16()(val a: Any) extends AnyVal // fail, was allowed 2.10.x
       ^
-14 errors found
+15 errors found