{{ directory }} in Twig template in base theme breaks subtheme

Question

Given a base theme called parent that is a child of stable, and a sub-theme called child whose base theme is parent, a Twig template implemented in the parent theme renders the wrong value for {{ directory }} variable.

1. In the parent theme, add a node.html.twig template with the following content:

   {# contents of parent/templates/node.html.twig #}
   file is themes/custom/parent/templates/content/node.html.twig<br>
   directory is: {{ directory }}
   

   Leave node.html.twig unimplemented in child theme, so that it inherits from parent.

2. Set default theme to child.

3. Visiting node/1 will print:

   file is themes/custom/parent/templates/content/node.html.twig
   directory is themes/custom/child
   

In theory, step 3 should produce a value of themes/custom/parent for the directory variable, otherwise, how can you depend on that variable to reference URLs of additional assets from the parent theme?

In actuality, {{ directory }} is acting exactly like {{ active_theme_path() }}.

As I understand it, this basically means that you cannot use {{ directory }} to reference any static assets in a base or parent theme, such as the path to an image icon, or using the {{ source() }} Twig function to insert an inline SVG. In my case, I'm trying to do just this:

{{ source(directory ~ '/images/svg/resources.svg') }}

But I get: Twig_Error_Loader: Template "themes/custom/child/images/svg/resources.svg" is not defined. Obviously, that file only exists in the parent theme.

Possibly this should be filed as a bug on drupal.org but wanted to check here first if this is already known and/or to award some karma points for a successful workaround.

Update:

My current workaround is to implement hook_preprocess() and set a global template variable called parent_theme_path that points to the parent theme path.

// @file parent.theme
/**
 * Prepares global variables for all templates.
 */
function parent_preprocess(&$variables) {
  $handler = \Drupal::service('theme_handler');
  $variables['parent_theme_path'] = $handler->getTheme('parent')->getPath();
}

{# @file parent/templates/node.html.twig #}
{{ source( parent_theme_path ~ 'images/svg/resources.svg') }}

Answer 1

You can also use this:

{{ source('@parent/../images/svg/resources.svg') }}

where @parent is the name of the parent theme. @parent references to the templates folder, so @parent/../ was added to go to the root directory of your theme.