[1.8.x] Fixed small inconsistency when handling aggregate's default_alias.

Refs #14030.

Backport of d450af8a26 from master

django/db/models/query.py

@@ -288,7 +288,13 @@ class QuerySet(object):
         if self.query.distinct_fields:
             raise NotImplementedError("aggregate() + distinct(fields) not implemented.")
         for arg in args:
-            if not hasattr(arg, 'default_alias'):
+            # The default_alias property may raise a TypeError, so we use
+            # a try/except construct rather than hasattr in order to remain
+            # consistent between PY2 and PY3 (hasattr would swallow
+            # the TypeError on PY2).
+            try:
+                arg.default_alias
+            except (AttributeError, TypeError):
                 raise TypeError("Complex aggregates require an alias")
             kwargs[arg.default_alias] = arg
 
@@ -759,14 +765,16 @@ class QuerySet(object):
         """
         annotations = OrderedDict()  # To preserve ordering of args
         for arg in args:
+            # The default_alias property may raise a TypeError, so we use
+            # a try/except construct rather than hasattr in order to remain
+            # consistent between PY2 and PY3 (hasattr would swallow
+            # the TypeError on PY2).
             try:
-                # we can't do an hasattr here because py2 returns False
-                # if default_alias exists but throws a TypeError
                 if arg.default_alias in kwargs:
                     raise ValueError("The named annotation '%s' conflicts with the "
                                      "default name for another annotation."
                                      % arg.default_alias)
-            except AttributeError:  # default_alias
+            except (AttributeError, TypeError):
                 raise TypeError("Complex annotations require an alias")
             annotations[arg.default_alias] = arg
         annotations.update(kwargs)

tests/aggregation/tests.py

@@ -768,10 +768,12 @@ class ComplexAggregateTestCase(TestCase):
         self.assertEqual(b3.sums, Approximate(Decimal("383.69"), places=2))
 
     def test_complex_aggregations_require_kwarg(self):
-        with six.assertRaisesRegex(self, TypeError, 'Complex expressions require an alias'):
+        with six.assertRaisesRegex(self, TypeError, 'Complex annotations require an alias'):
             Author.objects.annotate(Sum(F('age') + F('friends__age')))
         with six.assertRaisesRegex(self, TypeError, 'Complex aggregates require an alias'):
             Author.objects.aggregate(Sum('age') / Count('age'))
+        with six.assertRaisesRegex(self, TypeError, 'Complex aggregates require an alias'):
+            Author.objects.aggregate(Sum(Value(1)))
 
     def test_aggregate_over_complex_annotation(self):
         qs = Author.objects.annotate(