Specific preprocess functions for theme hook suggestions are not invoked

[klonos]

History:

• https://www.drupal.org/project/drupal/issues/939462 was raised in d.org against D7 core back in 2010, later moved to 8.x in 2011 and the issue marked as "Needs backport to D7".
• The fix was merged into core 8.x in 2015, and it was decided to raise https://www.drupal.org/project/drupal/issues/2563445 specifically for backporting the fix to 7.x.
• The D7 backport issue was closed in 2018 as "won't fix" (with the reason being given that "Drupal 7 is, likely, nearing EOL" 🙄)

Problem/Motivation

When you have a template suggestion available and are using that template, preprocess functions following naming conventions provided via hook_theme_suggestions_HOOK() are not being recognized, for example:

MYTHEME_preprocess_node__article()
MYTHEME_preprocess_block__search_form_block()
MYMODULE_preprocess_page__front()
MYMODULE_item_list__search_results()
etc.

Proposed resolution

Add a post-process step to the theme registry build to look for these types of preprocess functions and add them to the theme registry.

[klonos]

Other than the silliness of the D7 EoL reason, I'm opening this issue as a placeholder, to see whether the rest of the concerns around potential changes in contrib space are valid and to what extend, and whether we can possibly fix this in a non-disruptive way in Backdrop (and if not, then move it to 2.x). Specifically this remark:

Identify and discuss possible side effects.

There would likely be many. We couldn't even begin to think up how many or what kind, mainly due to the fact that this would be affecting hundreds of thousands of existing sites. Who knows what people have done to overcome this bug. ... Contrib has been "fixing" this for years, implementing this now would likely be majorly disruptive to existing sites as it would also likely require whatever contrib module that implemented said "fix" to reverse it and make a release. Thus inadvertently tying a core release with multiple contrib releases.