Observed behaviour

filter_overrides extra is not applied when the field has a choices attribute. I'm trying to use this to add a bootstrap class (form-select) to my select input, but the rendered select has no class.

Expected behaviour

filter_overrides extra is applied in all cases, and my bootstrap class makes it onto the rendered select element.

Cause

I think this stems from lines 406 and 407 of filterset.py:

if lookup_type == 'exact' and getattr(field, 'choices', None):
    return ChoiceFilter, {'choices': field.choices}

This ignores the params attribute which is defined on line 399:
params = data.get('extra', lambda field: {})(field)

Fix

I think we need to combine the params dict with the {'choices': field.choices} on line 407, e.g. change line 407 to return ChoiceFilter, {'choices': field.choices} | params (or {'choices': field.choices, **params} if you want to support Python versions <3.9). I'd suggest merging dictionaries in this order, as that would then also allow filter_overrides to override choices.

I've tested this change locally, and it resolves the issue.

I'm happy to make a pull request for that change if that would be of use?