is_404() - where does this get set?

Question

Where does is_404() get set?

Looking at the source in query.php:

function is_404() {
    global $wp_query;

    if ( ! isset( $wp_query ) ) {
        _doing_it_wrong( __FUNCTION__, __( 'Conditional query tags do not work before the query is run. Before then, they always return false.' ), '3.1' );
        return false;
    }

    return $wp_query->is_404();
}

which just returns a value already set in $wp_query.

However, if I then set a breakpoint in set_query_var this never gets hit.

Answer 1

There is a dedicated method for it WP_Query->set_404(). From quick search through code it can be set in WP_Query->parse_query() or WP->handle_404() (note WP, not WP_Query!).

But it can (and sometimes does) get set on query externally, for example by plugin that needs to invalidate specific resource.