Generate cleaner code for Unit-typed if-else expressions

Instead of generating an assignment to a Unit-typed variable, just add
the Unit value after the if-else expression:

    [async] scala.async.Async.async[Unit]({
      val f: scala.concurrent.Future[Int] = AnfTestClass.this.base(y);
      if (y.>(0))
        State.result_=(scala.async.Async.await[Int](f).+(2))
      else
        State.result_=(scala.async.Async.await[Int](f).-(2))
    })
    [async] ANF transform expands to:
    {
      val f: scala.concurrent.Future[Int] = AnfTestClass.this.base(y);
      if (y.>(0))
        {
          val await$8: Int = scala.async.Async.await[Int](f);
          State.result_=(await$8.+(2))
        }
      else
        {
          val await$9: Int = scala.async.Async.await[Int](f);
          State.result_=(await$9.-(2))
        };
      ()
    }

1 files changed, 20 insertions, 0 deletions

diff --git a/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala b/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala
index f38efa9..f2fc2d7 100644
--- a/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala
+++ b/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala
@@ -54,8 +54,20 @@ class AnfTestClass {
     }
     z + 1
   }
+
+  def futureUnitIfElse(y: Int): Future[Unit] = async {
+    val f = base(y)
+    if (y > 0) {
+      State.result = await(f) + 2
+    } else {
+      State.result = await(f) - 2
+    }
+  }
 }
 
+object State {
+  @volatile var result: Int = 0
+}
 
 @RunWith(classOf[JUnit4])
 class AnfTransformSpec {
@@ -91,4 +103,12 @@ class AnfTransformSpec {
     val res = Await.result(fut, 2 seconds)
     res mustBe (15)
   }
+
+  @Test
+  def `Unit-typed if-else in tail position`() {
+    val o = new AnfTestClass
+    val fut = o.futureUnitIfElse(10)
+    Await.result(fut, 2 seconds)
+    State.result mustBe (14)
+  }
 }