Solve error when coupons are applied to orders that reference deleted products (#46251)

Solve error when coupons are applied to orders that reference deleted products.

plugins/woocommerce/changelog/fix-27077-order-editor-coupons-deleted-products

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Addresses a fatal error that can occur when applying a coupon within the order editor (where one of the products has been deleted).

plugins/woocommerce/includes/class-wc-coupon.php

@@ -906,7 +906,7 @@ class WC_Coupon extends WC_Legacy_Coupon {
 	 * @return bool
 	 */
 	public function is_valid_for_product( $product, $values = array() ) {
-		if ( ! $this->is_type( wc_get_product_coupon_types() ) ) {
+		if ( ! $this->is_type( wc_get_product_coupon_types() ) || ! is_a( $product, WC_Product::class ) ) {
 			return apply_filters( 'woocommerce_coupon_is_valid_for_product', false, $product, $this, $values );
 		}
 

plugins/woocommerce/tests/php/includes/class-wc-coupon-test.php

@@ -0,0 +1,52 @@
+<?php
+
+/**
+ * Tests for WC_Coupon.
+ *
+ * See also ../../legacy/unit-tests/coupon/coupon.php for other related tests.
+ */
+class WC_Coupon_Tests extends WC_Unit_Test_Case {
+	/**
+	 * If a coupon is applied to an order where one or more products have been deleted, the operation should still
+	 * succeed.
+	 *
+	 * However, the coupon will have no impact on any line items referencing the deleted product(s), since in most cases
+	 * the product's eligibility can no longer be assessed (therefore, it is up to the merchant to manually adjust if
+	 * this is problematic).
+	 *
+	 * @see https://github.com/woocommerce/woocommerce/issues/27077
+	 *
+	 * @return void
+	 */
+	public function test_deleted_products_do_not_prevent_application_of_coupons(): void {
+		// Test order will have one product added already (price: 10, quantity: 4).
+		$order         = WC_Helper_Order::create_order();
+		$extra_product = WC_Helper_Product::create_simple_product();
+		$coupon        = WC_Helper_Coupon::create_coupon(
+			'look_after_the_pennies',
+			array(
+				'discount_type' => 'percent',
+				'coupon_amount' => 10,
+			)
+		);
+
+		// Add our further product to the order, but then delete the product itself.
+		$order->add_product( $extra_product );
+		$order->save();
+		wp_delete_post( $extra_product->get_id(), true );
+
+		$this->assertTrue(
+			$order->apply_coupon( $coupon ),
+			'The coupon was successfully applied to an order containing a deleted product, without triggering an error.'
+		);
+
+		// Both products have a cost of $10. The first item has a quantity of 4 units ($40). So, the 10% discount
+		// should give an actual discount total of $4 (the second line item is excluded from the calculation, because
+		// its product was deleted).
+		$this->assertEquals(
+			4,
+			$order->get_discount_total(),
+			'Line items associated with deleted products are not included in the discount calculation.'
+		);
+	}
+}