Avoid assigning null to vars of derived value type

`TreeGen#mkZero` returns `q"null"` for derived value classes.

```
scala> class V(val a: String) extends AnyVal
defined class V

scala> showRaw(gen.mkZero(typeOf[V]))
res0: String = Literal(Constant(null))
```

We use this API in async to generate the initial value for
ANF-lifted temporary variables.

However, this leads to NPEs, as after posterasure, we call the
unbox method on a null reference:

```
% cat sandbox/Macro.scala; scalac-hash v2.10.4 sandbox/Macro.scala; scala-hash v2.10.4 -e 'val x = Macros.myMacro'
import scala.reflect.macros.Context
import scala.language.experimental.macros

object Macros {
  def macroImpl(c: Context): c.Expr[C] = {
    import c.universe._
    val e1 = c.Expr[C](Literal(Constant(null)).setType(typeOf[C]))
    reify(e1.splice.asInstanceOf[C @annotation.unchecked.uncheckedVariance])
  }

  def myMacro: C = macro macroImpl
}

class C(val a: String) extends AnyVal
java.lang.NullPointerException
	at Main$$anon$1.<init>(scalacmd4059893593754060829.scala:1)
	at Main$.main(scalacmd4059893593754060829.scala:1)
	at Main.main(scalacmd4059893593754060829.scala)
```

This commit installs a custom version of `mkZero` that instead
returns `q"new C[$..targs](${mkZero(wrappedType)})`.

Thanks to @ewiner for pinpointing the problem.

1 files changed, 64 insertions, 0 deletions

diff --git a/src/test/scala/scala/async/run/toughtype/ToughType.scala b/src/test/scala/scala/async/run/toughtype/ToughType.scala
index 54a53c8..0f56ae0 100644
--- a/src/test/scala/scala/async/run/toughtype/ToughType.scala
+++ b/src/test/scala/scala/async/run/toughtype/ToughType.scala
@@ -228,8 +228,72 @@ class ToughTypeSpec {
       case `e` =>
     }
   }
+
+  @Test def ticket83ValueClass() {
+    import scala.async.Async._
+    import scala.concurrent._, duration._, ExecutionContext.Implicits.global
+    val f = async {
+      val uid = new IntWrapper("foo")
+      await(Future(uid))
+    }
+    val result = Await.result(f, 5.seconds)
+    result mustEqual (new IntWrapper("foo"))
+  }
+
+  @Test def ticket86NestedValueClass() {
+    import ExecutionContext.Implicits.global
+
+    val f = async {
+      val a = Future.successful(new IntWrapper("42"))
+      await(await(a).plusStr)
+    }
+    val result = Await.result(f, 5.seconds)
+    result mustEqual "42!"
+  }
+
+  @Test def ticket86MatchedValueClass(): Unit = {
+    import ExecutionContext.Implicits.global
+
+    def doAThing(param: IntWrapper) = Future(None)
+
+    val fut = async {
+      Option(new IntWrapper("value!")) match {
+        case Some(valueHolder) =>
+          await(doAThing(valueHolder))
+        case None =>
+          None
+      }
+    }
+
+    val result = Await.result(fut, 5.seconds)
+    result mustBe None
+  }
+
+  @Test def ticket86MatchedParameterizedValueClass(): Unit = {
+    import ExecutionContext.Implicits.global
+
+    def doAThing(param: ParamWrapper[String]) = Future(None)
+
+    val fut = async {
+      Option(new ParamWrapper("value!")) 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
+
+
 trait A
 
 trait B