Model field validator with a generic parameter causes infinite recursion when making migrations

[Michal Dabski]

Minimal code to reproduce (in models.py) in Django==4.2.25

from django.utils.deconstruct import deconstructible
from django.db import models

@deconstructible
class SchemaValidator:
    def __init__(self, expected_type: type):
        pass

    def __call__(self, *args, **kwargs):
        pass

class TestModel(models.Model):
    config: dict[str, float] = models.JSONField(validators=[SchemaValidator(dict[str, float])])

This triggers an error when running makemigrations:

$ ./manage.py makemigrations
C:\Users\Michal\src\mat-cms\.venv\Lib\site-packages\django_downloadview\__init__.py:4: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  import pkg_resources
Migrations for 'audit_builder':
  meg_forms\audit_builder\migrations\0083_alter_testmodel_config.py
    - Alter field config on testmodel
Traceback (most recent call last):
  File "C:\Users\Michal\src\mat-cms\.venv\Lib\site-packages\django\db\migrations\serializer.py", line 214, in serialize
    item_string, item_imports = serializer_factory(item).serialize()
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Michal\src\mat-cms\.venv\Lib\site-packages\django\db\migrations\serializer.py", line 214, in serialize
    item_string, item_imports = serializer_factory(item).serialize()
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Michal\src\mat-cms\.venv\Lib\site-packages\django\db\migrations\serializer.py", line 214, in serialize
    item_string, item_imports = serializer_factory(item).serialize()
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 996 more times]
  File "C:\Users\Michal\src\mat-cms\.venv\Lib\site-packages\django\db\migrations\serializer.py", line 389, in serializer_factory
    if isinstance(value, type_):
       ^^^^^^^^^^^^^^^^^^^^^^^^
RecursionError: maximum recursion depth exceeded

Debugging locally shows that:

• removing generic parameters from the dict type passed to SchemaValidator does not trigger the error
• The loop happens in django.db.migrations.serializer.IterableSerializer.serialize, it keeps recursively invoking serializer_factory(item).serialize() with item='*dict[str, float]' (GenericAlias)