Fix #1567: Widen private constructor in value class

Private or protected constructors of value classes need to be widenened
to public in order to enable boxing anywhere.

Technically we should also do something about qualified private constructors, but since we
want to get rid of them anyway it's urgent.

Author: odersky

src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -32,6 +32,9 @@ import SymUtils._
  * in [[ElimErasedValueType]].
  * This is different from the implementation of value classes in Scala 2
  * (see SIP-15) which uses `asInstanceOf` which does not typecheck.
+ *
+ * Finally, if the constructor of a value class is private pr protected
+ * it is widened to public.
  */
 class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with FullParameterization { thisTransformer =>
 
@@ -96,11 +99,18 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
         case _ =>
           moduleClassSym
       }
-    case ref: SymDenotation
-    if isMethodWithExtension(ref) && ref.hasAnnotation(defn.TailrecAnnot) =>
-      val ref1 = ref.copySymDenotation()
-      ref1.removeAnnotation(defn.TailrecAnnot)
-      ref1
+    case ref: SymDenotation =>
+      if (isMethodWithExtension(ref) && ref.hasAnnotation(defn.TailrecAnnot)) {
+        val ref1 = ref.copySymDenotation()
+        ref1.removeAnnotation(defn.TailrecAnnot)
+        ref1
+      }
+      else if (ref.isConstructor && isDerivedValueClass(ref.owner) && ref.is(AccessFlags)) {
+        val ref1 = ref.copySymDenotation()
+        ref1.resetFlag(AccessFlags)
+        ref1
+      }
+      else ref
     case _ =>
       ref
   }

tests/pos/1567/PosZInt_1.scala

@@ -0,0 +1,6 @@
+final class PosZInt private (val value: Int) extends AnyVal
+
+object PosZInt {
+  def from(value: Int): Option[PosZInt] =
+    if (value >= 0) Some(new PosZInt(value)) else None
+}

tests/pos/1567/Test_2.scala

@@ -0,0 +1,3 @@
+object Test {
+  scala.util.Try(PosZInt.from(1).get)
+}