sql-drop argument '--result-file' not working

[Luxian]

The documentation is quite confusing, drush help sql-drop describes the argument as:

--result-file=</path/to/file> Save to a file. The file should be relative to Drupal root. Recommended.

To my understanding if you use that argument, a dump of the database will be saved before running drop on all tables. Saving afterwards seems useless.

But running drush @alias sql-drop --result-file=backup.sql will not generate any file (v8.1.9).

In the code the--result-file doesn't appear to be used:

• command invoke
• base/mysql class implementation

I'm not sure what's the right approach here. Should that argument work? Or it was documented by mistake?

PS: I would like to use this as a default options for some aliases (like @prod) as a mitigation for something I cannot accomplish with policy.drush.php (issue #1708452).