Avoid catching fatal errors as a Failed future

scala-async 0.9.6 regressed and started catching fatal exceptions.

For 1.0.0, we're fixing this regression and harmonizing the behaviour
with direct use of the Future combinators.

Author: retronym

src/main/scala/scala/async/FutureStateMachine.scala

@@ -15,6 +15,7 @@ import java.util.Objects
 
 import scala.util.{Failure, Success, Try}
 import scala.concurrent.{ExecutionContext, Future, Promise}
+import scala.util.control.NonFatal
 
 /** The base class for state machines generated by the `scala.async.Async.async` macro.
  *  Not intended to be directly extended in user-written code.
@@ -34,15 +35,14 @@ abstract class FutureStateMachine(execContext: ExecutionContext) extends Functio
   /** Assign `i` to the state variable */
   protected def state_=(s: Int): Unit = state$async = s
 
+  NonFatal // eagerly classloading NonFatal to reduce the chance of a cascading StackOverflowError in `completeFailure`
+
   /** Complete the state machine with the given failure. */
-  protected def completeFailure(t: Throwable): Unit = {
-    //
-    // TODO https://github.com/scala/scala-async/issues/243
-    //
-    // scala-async accidentally started catching NonFatal exceptions in:
-    //  https://github.com/scala/scala-async/commit/e3ff0382ae4e015fc69da8335450718951714982#diff-136ab0b6ecaee5d240cd109e2b17ccb2R411
-    // This follows the new behaviour but should we fix the regression?
-    result$async.complete(Failure(t))
+  protected def completeFailure(t: Throwable): Unit = t match {
+    case NonFatal(t) =>
+      result$async.complete(Failure(t))
+    case _ =>
+      throw t
   }
 
   /** Complete the state machine with the given value. */

src/test/scala/scala/async/ExceptionalTest.scala

@@ -18,7 +18,6 @@ class ExceptionalTest {
   }
 
   @Test
-  @Ignore // TODO https://github.com/scala/scala-async/issues/243
   def nonFatalNotCaughtAsync(): Unit = {
     check { implicit ec =>
       async {