docs: @Trivial // traced by caller
      static <U> CompletionStage<U> failedStage(Throwable x, Executor executor) {
          if (JAVA8) {
              CompletableFuture<U> failedFuture = new CompletableFuture<U>();
              failedFuture.completeExceptionally(x);
              return new ManagedCompletionStage<U>(failedFuture, executor, null);
          } else {
              ManagedCompletableFuture<U> stage = new ManagedCompletionStage<U>(executor);
              stage.super_completeExceptionally(x);
              return stage;
          }
      }