docs: public static <T> CompletableFuture<T> toCompletableFuture(T t) {
          if (t instanceof CompletionStage) {
              //noinspection unchecked
              return ((CompletionStage<T>) t).toCompletableFuture();
          } else {
              return CompletableFuture.completedFuture(t);
          }
      }