Fixed #25307 -- Fixed QuerySet.annotate() crash with conditional expressions.

Thanks Travis Newport for the tests and Josh Smeaton for contributing
to the patch.

tests/aggregation_regress/tests.py

@@ -9,7 +9,8 @@ from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import FieldError
 from django.db import connection
 from django.db.models import (
-    Avg, Count, F, Max, Q, StdDev, Sum, Value, Variance,
+    Avg, Case, Count, DecimalField, F, IntegerField, Max, Q, StdDev, Sum,
+    Value, Variance, When,
 )
 from django.test import TestCase, skipUnlessAnyDBFeature, skipUnlessDBFeature
 from django.test.utils import Approximate
@@ -371,6 +372,51 @@ class AggregationTests(TestCase):
             {'c__max': 3}
         )
 
+    def test_conditional_aggreate(self):
+        # Conditional aggregation of a grouped queryset.
+        self.assertEqual(
+            Book.objects.annotate(c=Count('authors')).values('pk').aggregate(test=Sum(
+                Case(When(c__gt=1, then=1), output_field=IntegerField())
+            ))['test'],
+            3
+        )
+
+    def test_sliced_conditional_aggregate(self):
+        self.assertEqual(
+            Author.objects.all()[:5].aggregate(test=Sum(Case(
+                When(age__lte=35, then=1), output_field=IntegerField()
+            )))['test'],
+            3
+        )
+
+    def test_annotated_conditional_aggregate(self):
+        annotated_qs = Book.objects.annotate(discount_price=F('price') * 0.75)
+        self.assertAlmostEqual(
+            annotated_qs.aggregate(test=Avg(Case(
+                When(pages__lt=400, then='discount_price'),
+                output_field=DecimalField()
+            )))['test'],
+            22.27, places=2
+        )
+
+    def test_distinct_conditional_aggregate(self):
+        self.assertEqual(
+            Book.objects.distinct().aggregate(test=Avg(Case(
+                When(price=Decimal('29.69'), then='pages'),
+                output_field=IntegerField()
+            )))['test'],
+            325
+        )
+
+    def test_conditional_aggregate_on_complex_condition(self):
+        self.assertEqual(
+            Book.objects.distinct().aggregate(test=Avg(Case(
+                When(Q(price__gte=Decimal('29')) & Q(price__lt=Decimal('30')), then='pages'),
+                output_field=IntegerField()
+            )))['test'],
+            325
+        )
+
     def test_decimal_aggregate_annotation_filter(self):
         """
         Filtering on an aggregate annotation with Decimal values should work.