Allow all error types to BaseForm.add_error() in documentation

[CarrotManMatt]

I have made things!

The documentation for the method BaseForm.add_error() says the following:

The error argument can be a single error, a list of errors, or a
dictionary that maps field names to lists of errors. An "error" can be
either a simple string or an instance of ValidationError with its
message attribute set and a "list or dictionary" can be an actual
list or dict or an instance of ValidationError with its
error_list or error_dict attribute set.

If error is a dictionary, the field argument must be None and
errors will be added to the fields that correspond to the keys of the
dictionary.

I have updated the stubs for this method to include the additional types of errors as a list or dict. And made sure that the dict type can only be passed when field is None

[UnknownPlatypus]

Would be nice to have a test in tests/assert_type showcasing what we should accept and what should be refused ?

You can take inspiration from tests/assert_type/http/test_request.py and run mypy --strict tests/assert_type && pyright tests/assert_type/ -p pyrightconfig.testcases.json from the root to check if everything pass (or the CI will do it)

[sobolevn]

I agree, let's add a test :)

[CarrotManMatt]

Due to the current implementation of ValidationError, the type system cannot inspect whether an instance's error_dict attribute is set and only allow the first overload signature of BaseForm.add_error() (with field required to be None).

I am also unaware of how to make these tests exhaustive by testing for the correct failing cases of field not set to None but error being a dictionary. Is there a assert_fails() equivalent to assert_type()?

[UnknownPlatypus]

Is there a assert_fails() equivalent to assert_type()?

Not to my knowledge, but I believe the recommanded way to handle this is just to add appropriate type-ignore comments on offending lines

[CarrotManMatt]

Cheers for the suggestion @UnknownPlatypus! I have now added these failure cases (still unable to test the introspection of a ValidationError instance's error_dict). So this PR is now complete from my side and ready for review.

[sobolevn]

Thanks!