wp-includes/class-wp-navigation-fallback.php:231Returns the classic menu with the slug primary if it exists.
WP_Term|null private static function get_nav_menu_with_primary_slug( $classic_nav_menus ) { foreach ( $classic_nav_menus as $classic_nav_menu ) { if ( 'primary' === $classic_nav_menu->slug ) { return $classic_nav_menu; } } return null; }Introduced in 6.3.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-navigation-fallback.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.