Add `Q.empty()` to check for nested empty Q objects like `Q(Q())`

[jonathan-golorry]

Empty Q objects (Q() or ~Q(), etc) evaluate to False, but Q(Q()) evaluates to True. This interferes with the shortcut on logical operations involving empty Q objects and can lead to bugs when trying to detect empty operations.

def q_any(iterable):
    q = Q()
    for element in iterable:
        q |= element
    if q:
        return q
    return Q(pk__in=[])
Item.objects.filter(q_any())  # no items
Item.objects.filter(q_any([Q()]))  # no items
Item.objects.filter(q_any([Q(Q())]))  # all items

Patch ​https://github.com/django/django/pull/14127 removes empty Q objects from args during Q object initialization.

Patch ​https://github.com/django/django/pull/14130 adds .empty() for detecting nested empty Q objects and uses that instead of boolean evaluation for shortcutting logical operations between empty Q objects. So Q(x=1) | Q(Q()) will now shortcut to Q(x=1) the same way Q(x=1) | Q() does. No simplifying is done during Q object initialization.

This requires https://code.djangoproject.com/ticket/32548 in order to prevent a regression in logical operations between query expressions and empty Q objects.

[Mariusz Felisiak]

Thanks for this proposition, however I don't think we should implicitly remove anything from args, even an empty Q() instances. This would be backward incompatible, inconsistent with the Node behavior, and can be handled in your code.

[jonathan-golorry]

Here is an alternate approach that adds a way to check for nested empty Q objects instead of making them impossible to create.