Fixed #24932 -- Added Cast database function.

Thanks Ian Foote for the initial patch.

tests/db_functions/test_cast.py

@@ -0,0 +1,24 @@
+from django.db import models
+from django.db.models.expressions import Value
+from django.db.models.functions import Cast
+from django.test import TestCase
+
+from .models import Author
+
+
+class CastTests(TestCase):
+    @classmethod
+    def setUpTestData(self):
+        Author.objects.create(name='Bob', age=1)
+
+    def test_cast_from_value(self):
+        numbers = Author.objects.annotate(cast_integer=Cast(Value('0'), models.IntegerField()))
+        self.assertEqual(numbers.get().cast_integer, 0)
+
+    def test_cast_from_field(self):
+        numbers = Author.objects.annotate(cast_string=Cast('age', models.CharField(max_length=255)),)
+        self.assertEqual(numbers.get().cast_string, '1')
+
+    def test_cast_from_python(self):
+        numbers = Author.objects.annotate(cast_float=Cast(0, models.FloatField()))
+        self.assertEqual(numbers.get().cast_float, 0.0)