Use case:

I'm extending the built-in WP_REST_Posts_Controller class to provide extra collection parameters / WP_Query arguments (sort by meta field, filter by P2P connected posts, etc.) for a custom post type endpoint.

AFAICT there is no way to do this without duplicating all of the code in WP_REST_Posts_Controller::get_items() as I need the current WP_REST_Request object to add the appropriate query arguments.