whenComplete overwrites original exception (it shouldn't)

[vsilaev]

Here is a code snippet (from SimpleCompletionStage):

                failure -> {
                    try {
                        action.accept(null, failure);
                        handleFailure(nextStage, failure);
                    } catch (Throwable e) {
                        handleFailure(nextStage, e);
                    }
                }

However, for CompletableFuture it's different:
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html#whenComplete-java.util.function.BiConsumer-

i.e. original exception is not overwritten for next stages.

So probably it makes sense to use handleFailure(nextStage, failure); to propagate original exception (as CompletableFuture does). Personally, in my own library I've added exception from handler as suppressed exception to failure and forward failure to next stages.

By the way, handle family of methods of CompletableFuture in fact overrides original exception (though, this is undocumented).