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()

Fixed #120

Author: retronym

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

@@ -48,17 +48,19 @@ private[async] trait AnfTransform {
           val stats :+ expr = anf.transformToList(tree)
           def statsExprUnit =
             stats :+ expr :+ api.typecheck(atPos(expr.pos)(Literal(Constant(()))))
+          def statsExpr_??? =
+            stats :+ expr :+ api.typecheck(atPos(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)
               val ref = gen.mkAttributedStableRef(valDef.symbol).setType(tree.tpe)
               val ref1 = if (ref.tpe =:= definitions.UnitTpe)
-                // https://github.com/scala/async/issues/74
-                // Use a cast to hide from "pure expression does nothing" error
-                //
-                // TODO avoid creating a ValDef for the result of this await to avoid this tree shape altogether.
-                // This will require some deeper changes to the later parts of the macro which currently assume regular
-                // tree structure around `await` calls.
+              // https://github.com/scala/async/issues/74
+              // Use a cast to hide from "pure expression does nothing" error
+              //
+              // TODO avoid creating a ValDef for the result of this await to avoid this tree shape altogether.
+              // This will require some deeper changes to the later parts of the macro which currently assume regular
+              // tree structure around `await` calls.
                 gen.mkCast(ref, definitions.UnitTpe)
               else ref
               stats :+ valDef :+ atPos(tree.pos)(ref1)
@@ -68,6 +70,8 @@ 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) {
+                statsExpr_???
               } else {
                 val varDef = defineVar(name.ifRes, expr.tpe, tree.pos)
                 def branchWithAssign(orig: Tree) = api.typecheck(atPos(orig.pos) {
@@ -88,8 +92,9 @@ 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) {
+                statsExpr_???
+              } else {
                 val varDef = defineVar(name.matchRes, expr.tpe, tree.pos)
                 def typedAssign(lhs: Tree) =
                   api.typecheck(atPos(lhs.pos)(Assign(Ident(varDef.symbol), mkAttributedCastPreservingAnnotations(lhs, tpe(varDef.symbol)))))

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

@@ -151,6 +151,7 @@ private[async] trait TransformUtils {
 
     val NonFatalClass = rootMirror.staticModule("scala.util.control.NonFatal")
     val Async_await   = asyncBase.awaitMethod(c.universe)(c.macroApplication.symbol).ensuring(_ != NoSymbol)
+    val IllegalStateExceptionClass = rootMirror.staticClass("java.lang.IllegalStateException")
   }
 
   // `while(await(x))` ... or `do { await(x); ... } while(...)` contain an `If` that loops;

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")
+  }
 }