Test and simplify ANF transform of Nothing-typed if/match

Author: retronym

src/compiler/scala/tools/nsc/transform/async/AnfTransform.scala

@@ -176,18 +176,10 @@ private[async] trait AnfTransform extends TransformUtils {
       }
     }
 
-    @tailrec
     private def transformMatchOrIf[T <: Tree](tree: Tree, needsResultVar: Boolean, nameSource: asyncNames.NameSource[TermName])(core: Symbol => T): Tree = {
-      // if type of if/match is Unit don't introduce assignment,
-      // but add Unit value to bring it into form expected by async transform
-      if (isUnitType(tree.tpe)) {
-        assignUnitType(core(NoSymbol))
-      } else if (tree.tpe =:= definitions.NothingTpe) {
-        currentStats += assignUnitType(core(NoSymbol))
-        localTyper.typedPos(tree.pos)(Throw(New(IllegalStateExceptionClass)))
-      } else if (isPatMatGeneratedJump(tree)) {
-        transformMatchOrIf(assignUnitType(tree), needsResultVar, nameSource)(core)
-      } else if (!needsResultVar) {
+      if (isPatMatGeneratedJump(tree)) assignUnitType(tree)
+
+      if (!needsResultVar || isUnitType(tree.tpe) || (tree.tpe =:= definitions.NothingTpe)) {
         core(NoSymbol)
       } else {
         val varDef = defineVar(nameSource(), tree.tpe, tree.pos)

test/junit/scala/tools/nsc/async/AnnotationDrivenAsync.scala

@@ -442,6 +442,24 @@ class AnnotationDrivenAsync {
     assertEquals(100, run(code))
   }
 
+  @Test
+  def testNothingTypedExpr(): Unit = {
+    val code =
+      """
+        |import scala.concurrent._, duration.Duration, ExecutionContext.Implicits.global
+        |import scala.tools.partest.async.Async.{async, await}
+        |import Future.{successful => f}
+        |
+        |object Test {
+        |  def test: Future[Throwable] = async { if ("".isEmpty) {await(f("")); throw new RuntimeException("boo!")} else ??? }.failed
+        |}
+        |""".stripMargin
+    run(code) match {
+      case re: RuntimeException => assert(re.getMessage == "boo!")
+      case _ => Assert.fail()
+    }
+  }
+
   // Handy to debug the compiler or to collect code coverage statistics in IntelliJ.
   @Test
   @Ignore