Ticket #18752: 18752.5.diff

wp-includes/link-template.php

@@ -122 +122 @@
                         $cats = get_the_category($post->ID);
                         if ( $cats ) {
                                 usort($cats, '_usort_terms_by_ID'); // order by ID
-                                $category = $cats[0]->slug;
-                                if ( $parent = $cats[0]->parent )
+                                $category_object = get_term( apply_filters( 'post_link_category', $cats[0], $cats, $post ), 'category' );
+                                $category = $category_object->slug;
+                                if ( $parent = $category_object->parent )
                                         $category = get_category_parents($parent, false, '/', true) . $category;
                         }
                         // show default category in permalinks, without