Require an active subscription for referral coupons

Reject a referral coupon when the advocate has no active or pending-cancel subscription. WooCommerce Subscriptions is required.

functions.php

<?php

add_filter( 'automatewoo/referrals/validate_coupon_for_user', 'my_store_validate_referral_subscription', 10, 3 );
add_filter( 'automatewoo/referrals/validate_coupon_for_guest', 'my_store_validate_referral_subscription', 10, 3 );

/**
 * Require the advocate to have an active subscription.
 *
 * @param true|WP_Error $valid       Existing validation result.
 * @param int|string    $customer    Customer identifier.
 * @param int           $advocate_id Advocate user ID.
 * @return true|WP_Error
 */
function my_store_validate_referral_subscription( $valid, $customer, $advocate_id ) {
	if ( is_wp_error( $valid ) ) {
		return $valid;
	}

	if ( empty( $customer ) || ! function_exists( 'wcs_get_users_subscriptions' ) ) {
		return new WP_Error( 'coupon_invalid', __( 'This coupon is invalid.', 'my-store' ) ); // phpcs:ignore WordPress.WP.I18n.TextDomainMismatch
	}

	foreach ( wcs_get_users_subscriptions( absint( $advocate_id ) ) as $subscription ) {
		if ( $subscription->has_status( array( 'active', 'pending-cancel' ) ) ) {
			return true;
		}
	}

	return new WP_Error( 'coupon_invalid', __( 'This coupon is invalid.', 'my-store' ) ); // phpcs:ignore WordPress.WP.I18n.TextDomainMismatch
}