[5.2.x] Fixed #36388 -- Made QuerySet.union() return self when called with no arguments.

Regression in 9cb8baa0c4fa2c10789c5c8b65f4465932d4d172.
Thank you to Antoine Humeau for the report and Simon Charette for the review.

Backport of 802baf5da5b8d8b44990a8214a43b951e7ab8b39 from main.

AUTHORS

@@ -237,6 +237,7 @@ answer newbie questions, and generally made Django that much better:
     Ciaran McCormick <ciaran@ciaranmccormick.com>
     Claude Paroz <claude@2xlibre.net>
     Clint Ecker
+    Colleen Dunlap <https://medium.com/@colleen85052>
     colin@owlfish.com
     Colin Wood <cwood06@gmail.com>
     Collin Anderson <cmawebsite@gmail.com>

django/db/models/query.py

@@ -1554,6 +1554,8 @@ class QuerySet(AltersData):
             if len(qs) == 1:
                 return qs[0]
             return qs[0]._combinator_query("union", *qs[1:], all=all)
+        elif not other_qs:
+            return self
         return self._combinator_query("union", *other_qs, all=all)
 
     def intersection(self, *other_qs):

docs/releases/5.2.2.txt

@@ -15,3 +15,6 @@ Bugfixes
 * Fixed a bug in Django 5.2 where subqueries using ``"pk"`` to reference models
   with a ``CompositePrimaryKey`` failed to raise ``ValueError`` when too many
   or too few columns were selected (:ticket:`36392`).
+
+* Fixed a regression in Django 5.2 that caused a crash when no arguments were
+  passed into ``QuerySet.union()`` (:ticket:`36388`).

tests/queries/test_qs_combinators.py

@@ -88,6 +88,13 @@ class QuerySetSetOperationTests(TestCase):
         qs3 = qs1.union(qs2)
         self.assertNumbersEqual(qs3[:1], [0])
 
+    def test_union_empty_slice(self):
+        qs = Number.objects.union()
+        self.assertNumbersEqual(qs[:1], [0])
+        qs = Number.objects.union(all=True)
+        self.assertNumbersEqual(qs[:1], [0])
+        self.assertNumbersEqual(qs.order_by("num")[0:], list(range(0, 10)))
+
     def test_union_all_none_slice(self):
         qs = Number.objects.filter(id__in=[])
         with self.assertNumQueries(0):