Fixed #30572 -- Prevented values()/values_list() on combined queryset from mutating the list of columns in querysets.

django/db/models/sql/compiler.py

@@ -426,6 +426,7 @@ class SQLCompiler:
                 # must have the same columns list. Set the selects defined on
                 # the query on all combined queries, if not already set.
                 if not compiler.query.values_select and self.query.values_select:
+                    compiler.query = compiler.query.clone()
                     compiler.query.set_values((
                         *self.query.extra_select,
                         *self.query.values_select,

tests/queries/test_qs_combinators.py

@@ -123,6 +123,9 @@ class QuerySetSetOperationTests(TestCase):
         self.assertEqual(reserved_name['order'], 2)
         reserved_name = qs1.union(qs1).values_list('name', 'order', 'id').get()
         self.assertEqual(reserved_name[:2], ('a', 2))
+        # List of columns can be changed.
+        reserved_name = qs1.union(qs1).values_list('order').get()
+        self.assertEqual(reserved_name, (2,))
 
     def test_union_with_two_annotated_values_list(self):
         qs1 = Number.objects.filter(num=1).annotate(