[4.2.x] Fixed #34750 -- Fixed QuerySet.count() when grouping by unused multi-valued annotations.

Thanks Toan Vuong for the report.
Thanks Simon Charette for the review.

Regression in 59bea9efd2.
Backport of c9b9a52edc from main

django/db/models/sql/query.py

@@ -482,6 +482,11 @@ class Query(BaseExpression):
                             annotation_mask |= expr.get_refs()
                     for aggregate in aggregates.values():
                         annotation_mask |= aggregate.get_refs()
+                    # Avoid eliding expressions that might have an incidence on
+                    # the implicit grouping logic.
+                    for annotation_alias, annotation in self.annotation_select.items():
+                        if annotation.get_group_by_cols():
+                            annotation_mask.add(annotation_alias)
                     inner_query.set_annotation_mask(annotation_mask)
 
             # Add aggregates to the outer AggregateQuery. This requires making

docs/releases/4.2.4.txt

@@ -15,3 +15,7 @@ Bugfixes
 
 * Fixed a regression in Django 4.2 that caused a crash when grouping by a
   reference in a subquery (:ticket:`34748`).
+
+* Fixed a regression in Django 4.2 that caused aggregation over query that
+  uses explicit grouping by multi-valued annotations to group against the wrong
+  columns (:ticket:`34750`).

tests/aggregation/tests.py

@@ -2165,6 +2165,47 @@ class AggregateAnnotationPruningTests(TestCase):
         self.assertEqual(sql.count("select"), 2, "Subquery wrapping required")
         self.assertNotIn("authors_count", sql)
 
+    def test_unused_aliased_aggregate_and_annotation_reverse_fk(self):
+        Book.objects.create(
+            name="b3",
+            publisher=self.p2,
+            pages=1000,
+            rating=4.2,
+            price=50,
+            contact=self.a2,
+            pubdate=datetime.date.today(),
+        )
+        qs = Publisher.objects.annotate(
+            total_pages=Sum("book__pages"),
+            good_book=Case(
+                When(book__rating__gt=4.0, then=Value(True)),
+                default=Value(False),
+            ),
+        )
+        self.assertEqual(qs.count(), 3)
+
+    def test_unused_aliased_aggregate_and_annotation_reverse_fk_grouped(self):
+        Book.objects.create(
+            name="b3",
+            publisher=self.p2,
+            pages=1000,
+            rating=4.2,
+            price=50,
+            contact=self.a2,
+            pubdate=datetime.date.today(),
+        )
+        qs = (
+            Publisher.objects.values("id", "name")
+            .annotate(total_pages=Sum("book__pages"))
+            .annotate(
+                good_book=Case(
+                    When(book__rating__gt=4.0, then=Value(True)),
+                    default=Value(False),
+                )
+            )
+        )
+        self.assertEqual(qs.count(), 3)
+
     def test_non_aggregate_annotation_pruned(self):
         with CaptureQueriesContext(connection) as ctx:
             Book.objects.annotate(