[4.0.x] Refs #33263 -- Added warning to BaseDeleteView when delete() method is overridden.

Follow up to 3a45fea083.
Backport of 6bc437c0d8 from main

tests/generic_views/test_edit.py

@@ -4,7 +4,10 @@ from django.test import SimpleTestCase, TestCase, override_settings
 from django.test.client import RequestFactory
 from django.urls import reverse
 from django.views.generic.base import View
-from django.views.generic.edit import CreateView, FormMixin, ModelFormMixin
+from django.views.generic.edit import (
+    CreateView, DeleteView, DeleteViewCustomDeleteWarning, FormMixin,
+    ModelFormMixin,
+)
 
 from . import views
 from .forms import AuthorForm
@@ -426,3 +429,21 @@ class DeleteViewTests(TestCase):
             res.context_data['form'].errors['confirm'],
             ['This field is required.'],
         )
+
+    # RemovedInDjango50Warning.
+    def test_delete_with_custom_delete(self):
+        class AuthorDeleteView(DeleteView):
+            model = Author
+
+            def delete(self, request, *args, **kwargs):
+                # Custom logic.
+                pass
+
+        msg = (
+            'DeleteView uses FormMixin to handle POST requests. As a '
+            'consequence, any custom deletion logic in '
+            'AuthorDeleteView.delete() handler should be moved to '
+            'form_valid().'
+        )
+        with self.assertWarnsMessage(DeleteViewCustomDeleteWarning, msg):
+            AuthorDeleteView()