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, 6 insertions, 1 deletions

diff --git a/src/main/scala/scala/async/internal/ExprBuilder.scala b/src/main/scala/scala/async/internal/ExprBuilder.scala
index f24e37e..164e85b 100644
--- a/src/main/scala/scala/async/internal/ExprBuilder.scala
+++ b/src/main/scala/scala/async/internal/ExprBuilder.scala
@@ -35,7 +35,12 @@ trait ExprBuilder {
     var stats: List[Tree]
 
     def statsAnd(trees: List[Tree]): List[Tree] = {
-      val body = adaptToUnit(stats)
+      val body = stats match {
+        case init :+ last if tpeOf(last) =:= definitions.NothingTpe =>
+          adaptToUnit(init :+ Typed(last, TypeTree(definitions.AnyTpe)))
+        case _ =>
+          adaptToUnit(stats)
+      }
       Try(body, Nil, adaptToUnit(trees)) :: Nil
     }