Fixed #34267 -- Fixed sliced QuerySet.union() crash.

Regression in 3d734c0.

Thanks Raphaël Stefanini for the report.

django/db/models/sql/compiler.py

@@ -532,15 +532,12 @@ def get_combinator_sql(self, combinator, all):
                         "ORDER BY not allowed in subqueries of compound statements."
                     )
         elif self.query.is_sliced and combinator == "union":
-            limit = (self.query.low_mark, self.query.high_mark)
             for compiler in compilers:
                 # A sliced union cannot have its parts elided as some of them
                 # might be sliced as well and in the event where only a single
                 # part produces a non-empty resultset it might be impossible to
                 # generate valid SQL.
                 compiler.elide_empty = False
-                if not compiler.query.is_sliced:
-                    compiler.query.set_limits(*limit)
         parts = ()
         for compiler in compilers:
             try:

tests/queries/test_qs_combinators.py

@@ -87,6 +87,15 @@ def test_union_combined_slice_compound_empty(self):
         qs3 = qs1.union(qs2)
         self.assertNumbersEqual(qs3.order_by("num")[2:3], [2])
 
+    def test_union_slice_index(self):
+        Celebrity.objects.create(name="Famous")
+        c1 = Celebrity.objects.create(name="Very famous")
+
+        qs1 = Celebrity.objects.filter(name="nonexistent")
+        qs2 = Celebrity.objects.all()
+        combined_qs = qs1.union(qs2).order_by("name")
+        self.assertEqual(combined_qs[1], c1)
+
     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)
@@ -319,10 +328,10 @@ def test_union_with_select_related_and_first(self):
         e1 = ExtraInfo.objects.create(value=7, info="e1")
         a1 = Author.objects.create(name="a1", num=1, extra=e1)
         Author.objects.create(name="a2", num=3, extra=e1)
-        base_qs = Author.objects.select_related("extra")
+        base_qs = Author.objects.select_related("extra").order_by()
         qs1 = base_qs.filter(name="a1")
         qs2 = base_qs.filter(name="a2")
-        self.assertEqual(qs1.union(qs2).first(), a1)
+        self.assertEqual(qs1.union(qs2).order_by("name").first(), a1)
 
     def test_union_with_first(self):
         e1 = ExtraInfo.objects.create(value=7, info="e1")
@@ -455,8 +464,7 @@ def test_exists_union(self):
             captured_sql,
         )
         self.assertEqual(
-            captured_sql.count(connection.ops.limit_offset_sql(None, 1)),
-            3 if connection.features.supports_slicing_ordering_in_compound else 1,
+            captured_sql.count(connection.ops.limit_offset_sql(None, 1)), 1
         )
 
     def test_exists_union_empty_result(self):