In woocommerce how do I retrieve all variations like get_available_variations but including invisible

Question

I want to retrieve all variations in a template file including hidden variations,

like $product->get_available_variations

I have placed the following in my theme function.php to extend the WC_Product_Variable class.

But when I call my new function $product->get_all_variations(); in my template (single-product\add-to-cart\variable-subscription.php) it gives an undefined method error.

Calling $product->get_available_variations(); works fine.

What have I missed?

add_action( 'init', 'register_custom_variation' );
function register_custom_variation () {
class WC_Product_Variable_Extension extends WC_Product_Variable {
    public function __construct( $product ) {
        parent::__construct( $product );
    }

    public function get_all_variations() {

        $variation_ids        = $this->get_children();
        $available_variations = array();

        if ( is_callable( '_prime_post_caches' ) ) {
            _prime_post_caches( $variation_ids );
        }

        foreach ( $variation_ids as $variation_id ) {

            $variation = wc_get_product( $variation_id );

            // Hide out of stock variations if 'Hide out of stock items from the catalog' is checked.
            if ( ! $variation || ! $variation->exists() || ( 'yes' === get_option( 'woocommerce_hide_out_of_stock_items' ) && ! $variation->is_in_stock() ) ) {
                continue;
            }

            $available_variations[] = $this->get_all_variation( $variation );
        }

        $available_variations = array_values( array_filter( $available_variations ) );

        return $available_variations;
    }
}

}