Fixed bug, when exception occurred on order with coupons cancel, made by guest after creating of customer account.

app/code/Magento/Sales/Model/Order/CustomerAssignment.php

@@ -0,0 +1,59 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Sales\Model\Order;
+
+use Magento\Sales\Api\Data\OrderInterface;
+use Magento\Sales\Api\OrderRepositoryInterface;
+use Magento\Customer\Api\Data\CustomerInterface;
+use Magento\Framework\Event\ManagerInterface;
+
+class CustomerAssignment
+{
+    /**
+     * @var ManagerInterface
+     */
+    private $eventManager;
+
+    /**
+     * @var OrderRepositoryInterface
+     */
+    private $orderRepository;
+
+    /**
+     * CustomerAssignment constructor.
+     *
+     * @param ManagerInterface $eventManager
+     * @param OrderRepositoryInterface $orderRepository
+     */
+    public function __construct(
+        ManagerInterface $eventManager,
+        OrderRepositoryInterface $orderRepository
+    ) {
+        $this->eventManager = $eventManager;
+        $this->orderRepository = $orderRepository;
+    }
+
+    /**
+     * @param OrderInterface $order
+     * @param CustomerInterface $customer
+     */
+    public function execute(OrderInterface $order, CustomerInterface $customer)/*: void*/
+    {
+        $order->setCustomerId($customer->getId());
+        $order->setCustomerIsGuest(false);
+        $this->orderRepository->save($order);
+
+        $this->eventManager->dispatch(
+            'sales_order_customer_assign_after',
+            [
+                'order'     => $order,
+                'customer'  => $customer
+            ]
+        );
+    }
+}

app/code/Magento/Sales/Observer/AssignOrderToCustomerObserver.php

@@ -12,6 +12,7 @@
 use Magento\Framework\Event\Observer;
 use Magento\Framework\Event\ObserverInterface;
 use Magento\Sales\Api\OrderRepositoryInterface;
+use Magento\Sales\Model\Order\CustomerAssignment;
 
 /**
  * Assign order to customer created after issuing guest order.
@@ -24,11 +25,22 @@ class AssignOrderToCustomerObserver implements ObserverInterface
     private $orderRepository;
 
     /**
+     * @var CustomerAssignment
+     */
+    private $assignmentService;
+
+    /**
+     * AssignOrderToCustomerObserver constructor.
+     *
      * @param OrderRepositoryInterface $orderRepository
+     * @param CustomerAssignment $assignmentService
      */
-    public function __construct(OrderRepositoryInterface $orderRepository)
-    {
+    public function __construct(
+        OrderRepositoryInterface $orderRepository,
+        CustomerAssignment $assignmentService
+    ) {
         $this->orderRepository = $orderRepository;
+        $this->assignmentService = $assignmentService;
     }
 
     /**
@@ -44,11 +56,8 @@ public function execute(Observer $observer)
         if (array_key_exists('__sales_assign_order_id', $delegateData)) {
             $orderId = $delegateData['__sales_assign_order_id'];
             $order = $this->orderRepository->get($orderId);
-            if (!$order->getCustomerId()) {
-                //if customer ID wasn't already assigned then assigning.
-                $order->setCustomerId($customer->getId());
-                $order->setCustomerIsGuest(0);
-                $this->orderRepository->save($order);
+            if (!$order->getCustomerId() && $customer->getId()) {
+                $this->assignmentService->execute($order, $customer);
             }
         }
     }

app/code/Magento/Sales/Test/Unit/Observer/AssignOrderToCustomerObserverTest.php

@@ -12,6 +12,7 @@
 use Magento\Framework\Event\Observer;
 use Magento\Sales\Api\Data\OrderInterface;
 use Magento\Sales\Api\OrderRepositoryInterface;
+use Magento\Sales\Model\Order\CustomerAssignment;
 use Magento\Sales\Observer\AssignOrderToCustomerObserver;
 use PHPUnit\Framework\TestCase;
 use PHPUnit_Framework_MockObject_MockObject;
@@ -27,6 +28,9 @@ class AssignOrderToCustomerObserverTest extends TestCase
     /** @var OrderRepositoryInterface|PHPUnit_Framework_MockObject_MockObject */
     protected $orderRepositoryMock;
 
+    /** @var CustomerAssignment | PHPUnit_Framework_MockObject_MockObject */
+    protected $assignmentMock;
+
     /**
      * Set Up
      */
@@ -35,7 +39,12 @@ protected function setUp()
         $this->orderRepositoryMock = $this->getMockBuilder(OrderRepositoryInterface::class)
             ->disableOriginalConstructor()
             ->getMock();
-        $this->sut = new AssignOrderToCustomerObserver($this->orderRepositoryMock);
+
+        $this->assignmentMock =  $this->getMockBuilder(CustomerAssignment::class)
+        ->disableOriginalConstructor()
+        ->getMock();
+
+        $this->sut = new AssignOrderToCustomerObserver($this->orderRepositoryMock, $this->assignmentMock);
     }
 
     /**
@@ -69,13 +78,14 @@ public function testAssignOrderToCustomerAfterGuestOrder($customerId)
         $orderMock->expects($this->once())->method('getCustomerId')->willReturn($customerId);
         $this->orderRepositoryMock->expects($this->once())->method('get')->with($orderId)
             ->willReturn($orderMock);
-        if (!$customerId) {
-            $this->orderRepositoryMock->expects($this->once())->method('save')->with($orderMock);
+
+        if ($customerId) {
+            $this->assignmentMock->expects($this->once())->method('execute')->with($orderMock, $customerMock);
             $this->sut->execute($observerMock);
-            return ;
+            return;
         }
 
-        $this->orderRepositoryMock->expects($this->never())->method('save')->with($orderMock);
+        $this->assignmentMock->expects($this->never())->method('execute');
         $this->sut->execute($observerMock);
     }
 

app/code/Magento/SalesRule/Observer/AssignCouponDataAfterOrderCustomerAssignObserver.php

@@ -0,0 +1,52 @@
+<?php
+
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+declare(strict_types=1);
+
+namespace Magento\SalesRule\Observer;
+
+use Magento\Framework\Event\Observer;
+use Magento\SalesRule\Model\Coupon\UpdateCouponUsages;
+use Magento\Sales\Api\Data\OrderInterface;
+use Magento\Framework\Event\ObserverInterface;
+
+class AssignCouponDataAfterOrderCustomerAssignObserver implements ObserverInterface
+{
+    const EVENT_KEY_CUSTOMER = 'customer';
+
+    const EVENT_KEY_ORDER    = 'order';
+
+    /**
+     * @var UpdateCouponUsages
+     */
+    private $updateCouponUsages;
+
+    /**
+     * AssignCouponDataAfterOrderCustomerAssign constructor.
+     *
+     * @param UpdateCouponUsages $updateCouponUsages
+     */
+    public function __construct(
+        UpdateCouponUsages $updateCouponUsages
+    ) {
+        $this->updateCouponUsages = $updateCouponUsages;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function execute(Observer $observer)
+    {
+        $event = $observer->getEvent();
+        /** @var OrderInterface $order */
+        $order = $event->getData(self::EVENT_KEY_ORDER);
+
+        if ($order->getCustomerId()) {
+            $this->updateCouponUsages->execute($order, true);
+        }
+    }
+}

app/code/Magento/SalesRule/etc/events.xml

@@ -24,4 +24,7 @@
     <event name="magento_salesrule_api_data_ruleinterface_load_after">
         <observer name="legacy_model_load" instance="Magento\Framework\EntityManager\Observer\AfterEntityLoad" />
     </event>
+    <event name="sales_order_customer_assign_after">
+        <observer name="sales_order_assign_customer_after" instance="Magento\SalesRule\Observer\AssignCouponDataAfterOrderCustomerAssignObserver" />
+    </event>
 </config>