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 3d734c09ff.

Thanks Stefan Hammer for the report.

django/db/models/sql/compiler.py

@@ -531,7 +531,6 @@ class SQLCompiler:
         compilers = [
             query.get_compiler(self.using, self.connection, self.elide_empty)
             for query in self.query.combined_queries
-            if not query.is_empty()
         ]
         if not features.supports_slicing_ordering_in_compound:
             for compiler in compilers:
@@ -546,6 +545,11 @@ class SQLCompiler:
         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 = ()

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)