Allow ThemeSuggestions::alter() to be more dynamic

You need to implement a @BootstrapAlter plugin in your sub-theme:

namespace Drupal\THEMENAME\Plugin\Alter;

use Drupal\bootstrap\Utility\Unicode;
use Drupal\bootstrap\Utility\Variables;

/**
 * Implements hook_theme_suggestions_alter().
 *
 * @BootstrapAlter("theme_suggestions")
 */
class ThemeSuggestions extends \Drupal\bootstrap\Plugin\Alter\ThemeSuggestions {

  /**
   * {@inheritdoc}
   */
  public function alter(&$suggestions, &$context1 = NULL, &$hook = NULL) {
    $variables = Variables::create($context1);
    // Ignore "input__search", otherwise let base theme handle suggestions.
    if (Unicode::strpos($variables['theme_hook_original'], 'input__search') === FALSE) {
      parent::alter($suggestions, $context1, $hook);
    }
  }

}

Actually, I suppose we could parse $variables['theme_hook_original'] and append them to the suggestion provided, like is done for links__dropbutton, e.g. input__form_control__search.

edit: the above is an easy workaround until this can be implemented though.

This entire alter hook needs to be refactored TBH. I'll upload a patch here shortly for you to test.

Better title

I've gone ahead and created a CR for this as well: https://www.drupal.org/node/2838155

Ignore #2, just apply the patch in #6.

The theme hook suggestion that should work now is:
input--form-control--search.html.twig

do i still need to keep src/Plugin/Alter/ThemeSuggestions.php in my sub-themes?

No, not unless you need to add/remove/change any suggestions (which it doesn't sound like you do).

thanks so much for quickly addressing this and for all your contributions in general to this great drupal theme.

You're quite welcome :D Just glad to help provide a solid base theme.

Typo in title (for prosperity sake) and expanded to include method (for searchability sake).

Ugh, too tired.