Add 'parent' to the list of valid tax classes for product variations

Commit d9f9e74bd added a check to `WC_Product::set_tax_class()` to only accept valid tax classes, but this created a bug for product variations as this type of product has an extra tax class called 'parent'.

This commit fixes this problem by adding a new method to `WC_Product` that returns a list of valid tax classes. `WC_Product_Variation` then override this method, returning another list including the tax class 'parent'.

Fixes #17024

includes/abstracts/abstract-wc-product.php

@@ -902,7 +902,7 @@ class WC_Product extends WC_Abstract_Legacy_Product {
 	public function set_tax_class( $class ) {
 		$class         = sanitize_title( $class );
 		$class         = 'standard' === $class ? '' : $class;
-		$valid_classes = WC_Tax::get_tax_class_slugs();
+		$valid_classes = $this->get_valid_tax_classes();
 
 		if ( ! in_array( $class, $valid_classes ) ) {
 			$class = '';
@@ -911,6 +911,15 @@ class WC_Product extends WC_Abstract_Legacy_Product {
 		$this->set_prop( 'tax_class', $class );
 	}
 
+	/**
+	 * Return an array of valid tax classes
+	 *
+	 * @return array valid tax classes
+	 */
+	protected function get_valid_tax_classes() {
+		return WC_Tax::get_tax_class_slugs();
+	}
+
 	/**
 	 * Set if product manage stock.
 	 *

includes/class-wc-product-variation.php

@@ -487,4 +487,16 @@ class WC_Product_Variation extends WC_Product_Simple {
 	public function variation_is_visible() {
 		return apply_filters( 'woocommerce_variation_is_visible', 'publish' === get_post_status( $this->get_id() ) && '' !== $this->get_price(), $this->get_id(), $this->get_parent_id(), $this );
 	}
+
+	/**
+	 * Return valid tax classes. Adds 'parent' to the default list of valid tax classes.
+	 *
+	 * @return array valid tax classes
+	 */
+	protected function get_valid_tax_classes() {
+		$valid_classes = WC_Tax::get_tax_class_slugs();
+		$valid_classes[] = 'parent';
+
+		return $valid_classes;
+	}
 }