docs: @Trivial // traced by caller
      static <U> CompletionStage<U> completedStage(U value, Executor executor) {
          if (JAVA8) {
              return new ManagedCompletionStage<U>(CompletableFuture.completedFuture(value), executor, null);
          } else {
              ManagedCompletableFuture<U> stage = new ManagedCompletionStage<U>(executor);
              stage.super_complete(value);
              return stage;
          }
      }