If a preprocess function is defined for a non-existing template Drupal assumes that template exists

Problem/Motivation

This problem causes template suggestions to fall back to the base template (e.g. node) instead of the next strongest suggestion. Imagine a situation when you want to have a common node--[view-mode].html.twig but also have a specific function the alters some variables which you want to separate from the base theme_preprocess_node function, e.g. theme_preprocess_node__node_type__view_mode. The node--[view-mode].html.twig template won't be used, node.html.twig will be used instead.

Long description

I stumbled upon this during development. I have 2 node types where both use the same custom view mode, Card in this case. As I'm utilizing twig inheritance I wanted to define three templates:

• node--card.html.twig (which extends node.html.twig)
• node--foo--card.html.twig (which extends node--card.html.twig)
• node--baz--card.html.twig (which extends node--card.html.twig)

I had to alter some variables in one of the templates, so I created a my_theme_preprocess_node__foo__card in the my_theme.theme.

Then I wanted to make some changes in node--card.html.twig so I temporarily removed node--foo--card.html.twig and that's when I noticed that nodes listed in a view that should be using the node--card.html.twig template are using node.html.twig instead.

After removing/commenting out my_theme_preprocess_node__foo__card in my_theme.theme the correct template was used.

Steps to reproduce

1. Standard profile installation
2. Create a view that lists articles with the teaser view mode
3. Enable twig debugging
4. Create a Bartik subtheme
5. Create a node--teaser.html.twig
6. In the subtheme's subtheme.theme file create a subtheme_preprocess_node__article__teaser function
7. Enable the subtheme
8. Navigate to the previously created view and check the browser devtools which template is being used
9. Delete/comment out the subtheme_preprocess_node__article__teaser function, rebuild the cache and check the view again, the correct template is being used

I guess the reason why this happens is that when Drupal is collecting the preprocess hooks it's looking for the hook_theme implementations and it's not checked if the preprocess is running for an existing template.

Proposed resolution

I think there is value in defining hook_preprocess_HOOK for very specific theme implementations without an existing template, but the template suggestion order should still be respected.

Remaining tasks

Figure out the solution.