Is callback mode required if there's no callback?

[kainino0x]

Callback modes default to 0 which is invalid (btw, we should explain this).
But is 0 valid if there's no callback to apply the mode to? (the callback pointer is nullptr)
Also, do we allow callbacks to be nullptr in all cases?

[lokokung]

So in my opinion, nullptr callbacks don't make too much sense especially since, IIRC, all the callbacks have a status that kind of needs to be checked to understand whether the async call succeeded. Naively, I was thinking that it could be useful for something like WaitAny(OnSubmittedWorkDone()), but the issue is that if, for example, there was a validation error on that call, the user might expect that the work is done when in reality it wasn't. For many of the other callbacks, it makes even less sense to have a nullptr callback since the actual result is only obtained via the callback.