[backport] Avoid dead code warnings in generated code.

If we blindly splicing `{..$stats, ..$generatedCode}`, and the last
expression in `$stats` is of type `Nothing`, we'll incur a dead
code warning when typechecking the block.

This commit:
  - introduces a helper method to augment user-written stats with
    synthetic code
  - Emit a try/finally in that code (so we advance the state, even if we
    are about to exit the state machine in the async-block global
    exception handler
  - Hide `Nothing` typed expressions from the dead code analysis
    by wrapping them in an `expr: Any`

Fixes #150 (the part reported in the comments, not the original ticket.)

1 files changed, 26 insertions, 0 deletions

diff --git a/src/test/scala/scala/async/package.scala b/src/test/scala/scala/async/package.scala
index 974a989..31f3d09 100644
--- a/src/test/scala/scala/async/package.scala
+++ b/src/test/scala/scala/async/package.scala
@@ -42,6 +42,32 @@ package object async {
     m.mkToolBox(options = compileOptions)
   }
 
+  import scala.tools.nsc._, reporters._
+  def mkGlobal(compileOptions: String = ""): Global = {
+    val source = """
+                   | class Test {
+                   |   def test = {
+                   |     import scala.async.Async._, scala.concurrent._, ExecutionContext.Implicits.global
+                   |     async {
+                   |       val opt = await(async(Option.empty[String => Future[Unit]]))
+                   |       opt match {
+                   |         case None =>
+                   |           throw new RuntimeException("case a")
+                   |         case Some(f) =>
+                   |           await(f("case b"))
+                   |       }
+                   |     }
+                   |   }
+                   | }
+                   | """.stripMargin
+    val settings = new Settings()
+    settings.processArgumentString(compileOptions)
+    settings.usejavacp.value = true
+    settings.embeddedDefaults(getClass.getClassLoader)
+    val reporter = new StoreReporter
+    new Global(settings, reporter)
+  }
+
   def scalaBinaryVersion: String = {
     val PreReleasePattern = """.*-(M|RC).*""".r
     val Pattern = """(\d+\.\d+)\..*""".r