Fixed #36251 -- Avoided mutating form Meta.fields in BaseInlineFormSet.

Signed-off-by: SaJH <wogur981208@gmail.com>

django/forms/models.py

@@ -1137,8 +1137,7 @@ class BaseInlineFormSet(BaseModelFormSet):
         # Add the inline foreign key field to form._meta.fields if it's defined
         # to make sure validation isn't skipped on that field.
         if self.form._meta.fields and self.fk.name not in self.form._meta.fields:
-            if isinstance(self.form._meta.fields, tuple):
-                self.form._meta.fields = list(self.form._meta.fields)
+            self.form._meta.fields = list(self.form._meta.fields)
             self.form._meta.fields.append(self.fk.name)
 
     def initial_form_count(self):

tests/model_formsets/tests.py

@@ -1687,9 +1687,10 @@ class ModelFormsetTest(TestCase):
 
             class Meta:
                 model = Book
-                fields = ("title",)
+                fields = ["title"]
 
         BookFormSet = inlineformset_factory(Author, Book, form=BookForm)
+        self.assertEqual(BookForm.Meta.fields, ["title"])
         data = {
             "book_set-TOTAL_FORMS": "3",
             "book_set-INITIAL_FORMS": "0",
@@ -1700,6 +1701,7 @@ class ModelFormsetTest(TestCase):
         }
         author = Author.objects.create(name="test")
         formset = BookFormSet(data, instance=author)
+        self.assertEqual(BookForm.Meta.fields, ["title"])
         self.assertEqual(
             formset.errors,
             [{}, {"__all__": ["Please correct the duplicate values below."]}, {}],