As a Wikipedia user I want the category search results to give me results for the searched category and its subcategories automatically.
This is part of the Top 20 wishes of the German community
It's currently possible to search incategory:X or -incategory:X, where X is a category name, to include or exclude results categorised in Category:X. However, this does not cover subcategories. This would obviously be useful, particularly on Commons where search is an important way to find media.
This could be a separate keyword, say incategorysub:X. We could also, in addition, extend incategory to specify recursion levels, with incategory:X defaulting to incategory:0:X (just the category specified - no recursion), and incategory:5:X meaning that we want to traverse 5 levels deep, and perhaps incategory:-1:X meaning the same as incategorysub:X.
Obviously, the usefulness of this feature would be enhanced by also addressing T143310 (basically, providing GUI access to advanced search features).