Dangerous behavior for queryset combinator

[wckao]

Although it is documented that after queryset combinator is used ( union / intersection / difference ), "only LIMIT, OFFSET, COUNT(*), ORDER BY, and specifying columns (i.e. slicing, count(), order_by(), and values()/values_list()) are allowed on the resulting QuerySet".

But when other unsupported filtering is used, it is simply ignored instead of throwing an exception, which can be dangerous and hard to discover/debug.

For example,

User.objects.filter(is_active=True).union(User.objects.filter(is_active=False)).filter(is_active=True)

will actually return all the User instead of just active user, without any warning or exception.
the last filter is simply discarded and ignored.

[Carlton Gibson]

Duplicate of #27995. Is fixed for Django 3.0.