Options with bool(false) default value aren't a real flag

[Sweetchuck]

Describe the bug

Default value of an option which is defined like this $options = ['a' => FALSE] cannot be overridden from drush.yml.

I think all the options has to be overridable from drush.yml.

Here are the descriptions of the meanings of the default values: https://github.com/consolidation/annotated-command#option-default-values

To Reproduce

<?php

class FooCommands {
  /**
   * @command foo:bar
   */
  public function (
    $options = [
      'a' => FALSE,
    ]
  ) {
    var_dump($options['a']);
  }

}

Content of drush.yml

command:
  foo:
    bar:
      options:
       a: true

Run drush foo:bar

System Configuration

Q	A
Drush version?	Drush Commandline Tool 9.5.2
Drupal version?	8.6 but not relevant
PHP version?	7.2.11 but not relevant
OS?	Linux

Use case

I would like run a Drush command like this drush site:install with the following drush.yml file:

command:
  site:
    install:
      options:
        existing-config: true

But the command.site.install.options.existing-config is totally ignored. Also the DRUSH_OPTIONS_EXISTING_CONFIG='true' drush site-install --existing-config command doesn't work. So I have to add it to CLI command, like this: drush site-install --existing-config.

List of commands

1. \Drush\Commands\OptionsCommands::optionsetProcBuild
2. \Drush\Commands\OptionsCommands::optionsetGetEditor
3. \Drush\Commands\config\ConfigPullCommands::pull
4. \Drush\Commands\core\CacheCommands::set
5. \Drush\Commands\core\DrupalDirectoryCommands::drupalDirectory
6. \Drush\Commands\core\DrupliconCommands::optionset
7. \Drush\Commands\core\InitCommands::initializeDrush
8. \Drush\Commands\core\RunserverCommands::runserver
9. \Drush\Commands\core\SiteInstallCommands::install
10. \Drush\Commands\core\UpdateDBCommands::updatedb
11. \Drush\Commands\help\HelpCommands::help
12. \Drush\Commands\help\ListCommands::helpList
13. \Drush\Commands\sql\SqlCommands::conf
14. \Drush\Commands\sql\SqlCommands::query
15. \Drush\Commands\sql\SqlCommands::dump
16. \Drush\Commands\sql\SqlSyncCommands::sqlsync

[greg-1-anderson]

Duplicate of https://github.com/drush-ops/drush/issues/2956