Fixed #35969 -- Disallowed specifying a USING clause for altered generated field.

PostgreSQL versions 16.5 and above no longer permit the use
of a USING clause when changing the type of a generated column.

django/db/backends/postgresql/schema.py

@@ -120,6 +120,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         return None
 
     def _using_sql(self, new_field, old_field):
+        if new_field.generated:
+            return ""
         using_sql = " USING %(column)s::%(type)s"
         new_internal_type = new_field.get_internal_type()
         old_internal_type = old_field.get_internal_type()

tests/migrations/test_operations.py

@@ -6415,6 +6415,37 @@ class OperationTests(OperationTestBase):
             operation.database_backwards(app_label, editor, new_state, project_state)
         self.assertColumnNotExists(f"{app_label}_pony", "modified_pink")
 
+    @skipUnlessDBFeature("supports_stored_generated_columns")
+    def test_generated_field_changes_output_field(self):
+        app_label = "test_gfcof"
+        operation = migrations.AddField(
+            "Pony",
+            "modified_pink",
+            models.GeneratedField(
+                expression=F("pink") + F("pink"),
+                output_field=models.IntegerField(),
+                db_persist=True,
+            ),
+        )
+        from_state, to_state = self.make_test_state(app_label, operation)
+        # Add generated column.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, from_state, to_state)
+        # Update output_field used in the generated field.
+        operation = migrations.AlterField(
+            "Pony",
+            "modified_pink",
+            models.GeneratedField(
+                expression=F("pink") + F("pink"),
+                output_field=models.DecimalField(decimal_places=2, max_digits=16),
+                db_persist=True,
+            ),
+        )
+        from_state = to_state.clone()
+        to_state = self.apply_operations(app_label, from_state, [operation])
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, from_state, to_state)
+
     @skipUnlessDBFeature("supports_stored_generated_columns")
     def test_add_generated_field_stored(self):
         self._test_add_generated_field(db_persist=True)