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

(cherry picked from commit 80aaf18)

src/main/scala/scala/async/internal/AnfTransform.scala

@@ -70,6 +70,9 @@ private[async] trait AnfTransform {
         val stats :+ expr = anf.transformToList(tree)
         def statsExprUnit =
           stats :+ expr :+ localTyper.typedPos(expr.pos)(Literal(Constant(())))
+        def statsExprThrow =
+          stats :+ expr :+ localTyper.typedPos(expr.pos)(Throw(Apply(Select(New(gen.mkAttributedRef(defn.IllegalStateExceptionClass)), nme.CONSTRUCTOR), Nil)))
+
         expr match {
           case Apply(fun, args) if isAwait(fun) =>
             val valDef = defineVal(name.await, expr, tree.pos)
@@ -90,6 +93,8 @@ private[async] trait AnfTransform {
             // but add Unit value to bring it into form expected by async transform
             if (expr.tpe =:= definitions.UnitTpe) {
               statsExprUnit
+            } else if (expr.tpe =:= definitions.NothingTpe) {
+              statsExprThrow
             } else {
               val varDef = defineVar(name.ifRes, expr.tpe, tree.pos)
               def branchWithAssign(orig: Tree) = localTyper.typedPos(orig.pos) {
@@ -110,8 +115,9 @@ private[async] trait AnfTransform {
             // but add Unit value to bring it into form expected by async transform
             if (expr.tpe =:= definitions.UnitTpe) {
               statsExprUnit
-            }
-            else {
+            } else if (expr.tpe =:= definitions.NothingTpe) {
+              statsExprThrow
+            } else {
               val varDef = defineVar(name.matchRes, expr.tpe, tree.pos)
               def typedAssign(lhs: Tree) =
                 localTyper.typedPos(lhs.pos)(Assign(Ident(varDef.symbol), mkAttributedCastPreservingAnnotations(lhs, varDef.symbol.tpe)))

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

@@ -83,6 +83,8 @@ private[async] trait TransformUtils {
 
     val NonFatalClass = rootMirror.staticModule("scala.util.control.NonFatal")
     val Async_await   = asyncBase.awaitMethod(global)(macroApplication.symbol).ensuring(_ != NoSymbol)
+    val IllegalStateExceptionClass = rootMirror.staticClass("java.lang.IllegalStateException")
+
   }
 
   def isSafeToInline(tree: Tree) = {

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) mustBe List("view(a$1)")
   }
+
+  @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")
+  }
 }