From e82cb7cdb93306b5fccf6aeef0e087999f46a016 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 14 Oct 2016 09:12:58 +0200
Subject: [PATCH] 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.
---
 .../dotc/transform/ExtensionMethods.scala     | 20 ++++++++++++++-----
 tests/pos/1567/PosZInt_1.scala                |  6 ++++++
 tests/pos/1567/Test_2.scala                   |  3 +++
 3 files changed, 24 insertions(+), 5 deletions(-)
 create mode 100644 tests/pos/1567/PosZInt_1.scala
 create mode 100644 tests/pos/1567/Test_2.scala

diff --git a/src/dotty/tools/dotc/transform/ExtensionMethods.scala b/src/dotty/tools/dotc/transform/ExtensionMethods.scala
index 62a21198d0d7..5ae4e8a54de9 100644
--- a/src/dotty/tools/dotc/transform/ExtensionMethods.scala
+++ b/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
   }
diff --git a/tests/pos/1567/PosZInt_1.scala b/tests/pos/1567/PosZInt_1.scala
new file mode 100644
index 000000000000..60b4061e672b
--- /dev/null
+++ b/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
+}
diff --git a/tests/pos/1567/Test_2.scala b/tests/pos/1567/Test_2.scala
new file mode 100644
index 000000000000..db6ca7070a33
--- /dev/null
+++ b/tests/pos/1567/Test_2.scala
@@ -0,0 +1,3 @@
+object Test {
+  scala.util.Try(PosZInt.from(1).get)
+}