[4.0.x] Fixed #33346 -- Fixed SimpleTestCase.assertFormsetError() crash on a formset named "form".

Thanks OutOfFocus4 for the report.

Regression in 456466d932.

Backport of cb383753c0 from main.

django/test/testcases.py

@@ -557,7 +557,7 @@ class SimpleTestCase(unittest.TestCase):
         # Search all contexts for the error.
         found_formset = False
         for i, context in enumerate(contexts):
-            if formset not in context:
+            if formset not in context or not hasattr(context[formset], 'forms'):
                 continue
             found_formset = True
             for err in errors:

docs/releases/4.0.1.txt

@@ -9,4 +9,6 @@ Django 4.0.1 fixes several bugs in 4.0.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 4.0 that caused a crash of
+  :meth:`~django.test.SimpleTestCase.assertFormsetError` on a formset named
+  ``form`` (:ticket:`33346`).

tests/test_utils/tests.py

@@ -13,7 +13,10 @@ from django.core.files.storage import default_storage
 from django.db import (
     IntegrityError, connection, connections, models, router, transaction,
 )
-from django.forms import EmailField, IntegerField
+from django.forms import (
+    CharField, EmailField, Form, IntegerField, ValidationError,
+    formset_factory,
+)
 from django.http import HttpResponse
 from django.template.loader import render_to_string
 from django.test import (
@@ -1241,6 +1244,44 @@ class AssertURLEqualTests(SimpleTestCase):
             )
 
 
+class TestForm(Form):
+    field = CharField()
+
+    def clean_field(self):
+        value = self.cleaned_data.get('field', '')
+        if value == 'invalid':
+            raise ValidationError('invalid value')
+        return value
+
+
+class TestFormset(formset_factory(TestForm)):
+    @classmethod
+    def _get_cleaned_formset(cls, field_value):
+        formset = cls({
+            'form-TOTAL_FORMS': '1',
+            'form-INITIAL_FORMS': '0',
+            'form-0-field': field_value,
+        })
+        formset.full_clean()
+        return formset
+
+    @classmethod
+    def invalid(cls):
+        return cls._get_cleaned_formset('invalid')
+
+
+class AssertFormsetErrorTests(SimpleTestCase):
+    def test_formset_named_form(self):
+        formset = TestFormset.invalid()
+        # The mocked context emulates the template-based rendering of the
+        # formset.
+        response = mock.Mock(context=[
+            {'form': formset},
+            {'form': formset.management_form},
+        ])
+        self.assertFormsetError(response, 'form', 0, 'field', 'invalid value')
+
+
 class FirstUrls:
     urlpatterns = [path('first/', empty_response, name='first')]