Fixed #34438 -- Reallowed extending UserCreationForm.

Regression in 298d02a77a.

tests/auth_tests/test_forms.py

@@ -372,6 +372,35 @@ class UserCreationFormTest(TestDataMixin, TestCase):
             ["A user with that username already exists."],
         )
 
+    @override_settings(AUTH_USER_MODEL="auth_tests.ExtensionUser")
+    def test_case_insensitive_username_custom_user_and_error_message(self):
+        class CustomUserCreationForm(UserCreationForm):
+            class Meta(UserCreationForm.Meta):
+                model = ExtensionUser
+                fields = UserCreationForm.Meta.fields + ("date_of_birth",)
+                error_messages = {
+                    "username": {"unique": "This username has already been taken."}
+                }
+
+        ExtensionUser.objects.create_user(
+            username="testclient",
+            password="password",
+            email="testclient@example.com",
+            date_of_birth=datetime.date(1984, 3, 5),
+        )
+        data = {
+            "username": "TeStClIeNt",
+            "password1": "test123",
+            "password2": "test123",
+            "date_of_birth": "1980-01-01",
+        }
+        form = CustomUserCreationForm(data)
+        self.assertIs(form.is_valid(), False)
+        self.assertEqual(
+            form["username"].errors,
+            ["This username has already been taken."],
+        )
+
 
 # To verify that the login form rejects inactive users, use an authentication
 # backend that allows them.