!str handle empty stream toFuture

akka-stream/src/main/scala/akka/stream/impl/FlowImpl.scala

@@ -74,7 +74,9 @@ private[akka] case class FlowImpl[I, O](producerNode: Ast.ProducerNode[I], ops:
 
   def toFuture(generator: ProcessorGenerator): Future[O] = {
     val p = Promise[O]()
-    transformRecover(0)((x, in) ⇒ { p complete in; 1 -> Nil }, isComplete = _ == 1).consume(generator)
+    transformRecover(0)((x, in) ⇒ { p complete in; 1 -> Nil },
+      onComplete = _ ⇒ { p.tryFailure(new NoSuchElementException("empty stream")); Nil },
+      isComplete = _ == 1).consume(generator)
     p.future
   }
 

akka-stream/src/test/scala/akka/stream/FlowToFutureSpec.scala

@@ -43,6 +43,19 @@ class FlowToFutureSpec extends AkkaSpec with ScriptedTest {
       f.value.get should be(Failure(ex))
     }
 
+    "yield NoSuchElementExcption for empty stream" in {
+      val p = StreamTestKit.producerProbe[Int]
+      val f = Flow(p).toFuture(gen)
+      val proc = p.expectSubscription
+      proc.expectRequestMore()
+      proc.sendComplete()
+      Await.ready(f, 100.millis)
+      f.value.get match {
+        case Failure(e: NoSuchElementException) ⇒ e.getMessage() should be("empty stream")
+        case x                                  ⇒ fail("expected NoSuchElementException, got " + x)
+      }
+    }
+
   }
 
 }