Fixed #28857 -- Fixed invalid SQL when using Cast with complex expressions on PostgreSQL.

django/db/models/functions/comparison.py

@@ -21,7 +21,9 @@ class Cast(Func):
 
     def as_postgresql(self, compiler, connection):
         # CAST would be valid too, but the :: shortcut syntax is more readable.
-        return self.as_sql(compiler, connection, template='%(expressions)s::%(db_type)s')
+        # 'expressions' is wrapped in parentheses in case it's a complex
+        # expression.
+        return self.as_sql(compiler, connection, template='(%(expressions)s)::%(db_type)s')
 
 
 class Coalesce(Func):

tests/db_functions/test_cast.py

 import datetime
 import decimal
+import unittest
 
-from django.db import models
+from django.db import connection, models
+from django.db.models import Avg
 from django.db.models.expressions import Value
 from django.db.models.functions import Cast
-from django.test import TestCase, ignore_warnings, skipUnlessDBFeature
+from django.test import (
+    TestCase, ignore_warnings, override_settings, skipUnlessDBFeature,
+)
 
 from .models import Author
 
@@ -60,3 +64,13 @@ class CastTests(TestCase):
         cast_float = numbers.get().cast_float
         self.assertIsInstance(cast_float, float)
         self.assertEqual(cast_float, 0.125)
+
+    @unittest.skipUnless(connection.vendor == 'postgresql', 'PostgreSQL test')
+    @override_settings(DEBUG=True)
+    def test_expression_wrapped_with_parentheses_on_postgresql(self):
+        """
+        The SQL for the Cast expression is wrapped with parentheses in case
+        it's a complex expression.
+        """
+        list(Author.objects.annotate(cast_float=Cast(Avg('age'), models.FloatField())))
+        self.assertIn('(AVG("db_functions_author"."age"))::double precision', connection.queries[-1]['sql'])