Fix #1567: Widen private constructor in value class

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)
+}