Improve error handling when complex annotations are not passed an alias

[Vinko Mlacic]

When complex annotations are not passed an alias, the default alias is usually used. If the annotate arg does not define the default alias, an error saying "Complex annotations require an alias" is raised.

This works fine with args that are subclasses of Aggregate subclass because the default_alias property on the Aggregate subclass raises that same TypeError.

However, when using expressions that are not Aggregate subclasses, this fails with an obscure message saying that default_alias is not a property on the expression object.

Code to reproduce:

models.py:

class Book(models.Model):
    name = models.CharField(max_length=255)

tests.py:

Book.objects.annotate(Value("title"))

Traceback (most recent call last):
  File "/usr/lib/python3.12/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/lib/python3.12/unittest/case.py", line 634, in run
    self._callTestMethod(testMethod)
  File "/usr/lib/python3.12/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
       ^^^^^^^^
  File "/home/vinko/projects/django/tests/annotations/tests.py", line 975, in test_complex_annotations_must_have_an_alias
    Book.objects.annotate(Value("title"))
  File "/home/vinko/projects/django/django/db/models/manager.py", line 87, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vinko/projects/django/django/db/models/query.py", line 1631, in annotate
    return self._annotate(args, kwargs, select=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vinko/projects/django/django/db/models/query.py", line 1648, in _annotate
    if arg.default_alias in kwargs:
       ^^^^^^^^^^^^^^^^^
AttributeError: 'Value' object has no attribute 'default_alias'

Similar happens with other non Aggregate types like: F, Case - When, Subquery, Exists, ...

[Vinko Mlacic]

Opened a PR with the changes here: ​https://github.com/django/django/pull/19113

[Vinko Mlacic]

It seems like the unwanted behavior originated from ed0cbc8d8b314e3b4a0305d0be3cf366d8ee4a74

[Sarah Boyce]

Thank you for the ticket and patch! Agree this needs better handling 👍