Avoid masking user exception with ??? for Nothing typed expressions

Code like:

    val x = if (cond) throw new A else throw new B

Was being transformed to:

    val ifRes = ???
    if (cond) ifRes = throw new A else ifRes = throw new B
    val x = ifRes

by way of the use of `gen.mkZero` which throws `???` if the requested type is `Nothing`

This commit special cases `Nothing` typed expressions in a similar manner to `Unit` type expressions.
The example above is now translated to:

    if (cond) throw new A else throw new B
    val x = throw new IllegalStateException()

Fixes #120

1 files changed, 42 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 2cce7e8..13cc351 100644
--- a/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala
+++ b/src/test/scala/scala/async/run/anf/AnfTransformSpec.scala
@@ -405,4 +405,46 @@ class AnfTransformSpec {
     val applyImplicitView = tree.collect { case x if x.getClass.getName.endsWith("ApplyImplicitView") => x }
     applyImplicitView.map(_.toString) mustStartWith List("view(a$macro$")
   }
+
+  @Test
+  def nothingTypedIf(): Unit = {
+    import scala.async.internal.AsyncId.{async, await}
+    val result = util.Try(async {
+      if (true) {
+        val n = await(1)
+        if (n < 2) {
+          throw new RuntimeException("case a")
+        }
+        else {
+          throw new RuntimeException("case b")
+        }
+      }
+      else {
+        "case c"
+      }
+    })
+
+    assert(result.asInstanceOf[util.Failure[_]].exception.getMessage == "case a")
+  }
+
+  @Test
+  def nothingTypedMatch(): Unit = {
+    import scala.async.internal.AsyncId.{async, await}
+    val result = util.Try(async {
+      0 match {
+        case _ if "".isEmpty =>
+          val n = await(1)
+          n match {
+            case _ if n < 2 =>
+              throw new RuntimeException("case a")
+            case _ =>
+              throw new RuntimeException("case b")
+          }
+        case _ =>
+          "case c"
+      }
+    })
+
+    assert(result.asInstanceOf[util.Failure[_]].exception.getMessage == "case a")
+  }
 }