What require_wp_db function in Wordpress would look like, if it worked

require_wp_db_patch.php

<?php
function require_wp_db() {
    global $wpdb;
    if ( isset($wpdb) ){
        return;
    }
    // Should check for override db.php file first
    // - including both means fatal error on duplicate class
    if ( file_exists( WP_CONTENT_DIR . '/db.php' ) ){
        require_once( WP_CONTENT_DIR . '/db.php' );
    }
    // include core default
    else {
        require_once( ABSPATH . WPINC . '/wp-db.php' );
    }
    // The db.php adapter may have defined the global for us
    if( ! isset($wpdb) ){
        $wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );
    }
}

Sorry, got tripped up by the comment editor there.