[5.0.x] Fixed #34946 -- Preserved db_default on combined default field addition.

Regression in 7414704e88.
Backport of 8a28e983df from main

tests/schema/tests.py

@@ -2263,6 +2263,23 @@ class SchemaTests(TransactionTestCase):
         columns = self.column_classes(AuthorDbDefault)
         self.assertEqual(columns["renamed_year"][1].default, "1985")
 
+    @isolate_apps("schema")
+    def test_add_field_both_defaults_preserves_db_default(self):
+        class Author(Model):
+            class Meta:
+                app_label = "schema"
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+
+        field = IntegerField(default=1985, db_default=1988)
+        field.set_attributes_from_name("birth_year")
+        field.model = Author
+        with connection.schema_editor() as editor:
+            editor.add_field(Author, field)
+        columns = self.column_classes(Author)
+        self.assertEqual(columns["birth_year"][1].default, "1988")
+
     @skipUnlessDBFeature(
         "supports_column_check_constraints", "can_introspect_check_constraints"
     )