Additional tests and comments around mkZero for value classes

Author: retronym

src/main/scala/scala/async/internal/TransformUtils.scala

@@ -333,10 +333,20 @@ private[async] trait TransformUtils {
   def mkZero(tp: Type): Tree = {
     if (tp.typeSymbol.asClass.isDerivedValueClass) {
       val argZero = mkZero(derivedValueClassUnbox(tp.typeSymbol).infoIn(tp).resultType)
+      val baseType = tp.baseType(tp.typeSymbol) // use base type here to dealias / strip phantom "tagged types" etc.
+
+      // By explicitly attributing the types and symbols here, we subvert privacy.
+      // Otherwise, ticket86PrivateValueClass would fail.
+
+      // Approximately:
+      // q"new ${valueClass}[$..targs](argZero)"
       val target: Tree = gen.mkAttributedSelect(
         c.typecheck(atMacroPos(
-        New(TypeTree(tp.baseType(tp.typeSymbol))))), tp.typeSymbol.asClass.primaryConstructor)
+        New(TypeTree(baseType)))), tp.typeSymbol.asClass.primaryConstructor)
+
       val zero = gen.mkMethodCall(target, argZero :: Nil)
+
+      // restore the original type which we might otherwise have weakened with `baseType` above
       gen.mkCast(zero, tp)
     } else {
       gen.mkZero(tp)

src/test/scala/scala/async/run/toughtype/ToughType.scala

@@ -286,13 +286,37 @@ class ToughTypeSpec {
     val result = Await.result(fut, 5.seconds)
     result mustBe None
   }
+
+  @Test def ticket86PrivateValueClass(): Unit = {
+    import ExecutionContext.Implicits.global
+
+    def doAThing(param: PrivateWrapper) = Future(None)
+
+    val fut = async {
+      Option(PrivateWrapper.Instance) match {
+        case Some(valueHolder) =>
+          await(doAThing(valueHolder))
+        case None =>
+          None
+      }
+    }
+
+    val result = Await.result(fut, 5.seconds)
+    result mustBe None
+  }
 }
 
 class IntWrapper(val value: String) extends AnyVal {
   def plusStr = Future.successful(value + "!")
 }
 class ParamWrapper[T](val value: T) extends AnyVal
 
+class PrivateWrapper private (private val value: String) extends AnyVal
+object PrivateWrapper {
+  def Instance = new PrivateWrapper("")
+}
+
+
 trait A
 
 trait B