Fixed #31580 -- Added error messages on distinct() following union(), intersection(), and difference().

django/db/models/query.py

@@ -1138,6 +1138,7 @@ class QuerySet:
         """
         Return a new QuerySet instance that will select only distinct results.
         """
+        self._not_support_combined_queries('distinct')
         assert not self.query.is_sliced, \
             "Cannot create distinct fields once a slice has been taken."
         obj = self._chain()

tests/queries/test_qs_combinators.py

@@ -272,6 +272,7 @@ class QuerySetSetOperationTests(TestCase):
                 'annotate',
                 'defer',
                 'delete',
+                'distinct',
                 'exclude',
                 'extra',
                 'filter',