Fixed #34125 -- Fixed sliced QuerySet.union() crash on a single non-empty queryset.

The bug existed since sliced query union was added but was elevated to
query union slices by moving the .exists() optimization to the compiler
in 3d734c09ff0138441dfe0a59010435871d17950f.

Thanks Stefan Hammer for the report.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/queries/test_qs_combinators.py b/tests/queries/test_qs_combinators.py
index 97b3f97b68..865e172816 100644
--- a/tests/queries/test_qs_combinators.py
+++ b/tests/queries/test_qs_combinators.py
@@ -61,6 +61,32 @@ class QuerySetSetOperationTests(TestCase):
         self.assertSequenceEqual(qs3.none(), [])
         self.assertNumbersEqual(qs3, [0, 1, 8, 9], ordered=False)
 
+    def test_union_none_slice(self):
+        qs1 = Number.objects.filter(num__lte=0)
+        qs2 = Number.objects.none()
+        qs3 = qs1.union(qs2)
+        self.assertNumbersEqual(qs3[:1], [0])
+
+    def test_union_empty_filter_slice(self):
+        qs1 = Number.objects.filter(num__lte=0)
+        qs2 = Number.objects.filter(pk__in=[])
+        qs3 = qs1.union(qs2)
+        self.assertNumbersEqual(qs3[:1], [0])
+
+    @skipUnlessDBFeature("supports_slicing_ordering_in_compound")
+    def test_union_slice_compound_empty(self):
+        qs1 = Number.objects.filter(num__lte=0)[:1]
+        qs2 = Number.objects.none()
+        qs3 = qs1.union(qs2)
+        self.assertNumbersEqual(qs3[:1], [0])
+
+    @skipUnlessDBFeature("supports_slicing_ordering_in_compound")
+    def test_union_combined_slice_compound_empty(self):
+        qs1 = Number.objects.filter(num__lte=2)[:3]
+        qs2 = Number.objects.none()
+        qs3 = qs1.union(qs2)
+        self.assertNumbersEqual(qs3.order_by("num")[2:3], [2])
+
     def test_union_order_with_null_first_last(self):
         Number.objects.filter(other_num=5).update(other_num=None)
         qs1 = Number.objects.filter(num__lte=1)