Include custom post types in categories widget

So I got a fairly simple setup, one CPT with the default category and post_tag as taxonomies.

Say I have 1 post and 1 CPT entry assigned to the same category. I need this to be reflected inside the category widget as well.

I’ve managed to display both of them on the category page via the request filter so the links generated by the widget work.

The problem is that if the post didn’t exist, the widget would not generate a link for the category (considering it empty).

Also, the item count is incorrect, 1 instead of 2.

Digging through the widget code, this is what’s causing the problems:

wp_list_categories(apply_filters('widget_categories_args', $cat_args));

So I can alter the arguments via the filter but wp_list_categories() doesn’t have an argument for what post type it should use.

I’ve seen someone else encounter the problem before:

http://wordpress.org/support/topic/custom-post-types-and-category-widget – no luck

http://themehybrid.com/support/topic/custom-post-types-and-category-widget – same guy, can’t read Justin’s responses but I assume no luck again. he filed a ticket at the end which got closed

Thoughts on the solution

I’d really like to not copy/paste the default widget code and create a new one but solve this using a filter somehow.

That being said, would a custom walker help with this? Any ideas?

As a sidenote, I solved a similar issue with the Archives widget using this:

/*
* Add CPTs to wp_get_archives()
* http://bajada.net/2010/07/15/adding-custom-post-types-to-wp_get_archives
*/
function ucc_getarchives_where_filter( $where , $r ) {
    $args = array( 'public' => true , '_builtin' => false );
    $output = 'names'; $operator = 'and';

    $post_types = get_post_types( $args , $output , $operator );
    $post_types = array_merge( $post_types , array( 'post' ) ); $post_types = "'" . implode( "' , '" , $post_types ) . "'";

    return str_replace( "post_type = 'post'" , "post_type IN ( $post_types )" , $where );
}
add_filter( 'getarchives_where' , 'ucc_getarchives_where_filter' , 10 , 2 ); 

Cheers!