magento/magento2#39884: Fix type compatibility in Invoice comments me…

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

@@ -713,7 +713,7 @@ public function addComment($comment, $notify = false, $visibleOnFront = false)
         }
         $comment->setInvoice($this)->setStoreId($this->getStoreId())->setParentId($this->getId());
         if (!$comment->getId()) {
-            $this->getCommentsCollection()->addItem($comment);
+            $this->getCommentsCollection(true)->addItem($comment);
         }
         $this->_hasDataChanges = true;
         return $this;
@@ -727,24 +727,28 @@ public function addComment($comment, $notify = false, $visibleOnFront = false)
      */
     public function getCommentsCollection($reload = false)
     {
-        if (!$this->hasData(InvoiceInterface::COMMENTS) || $reload) {
+        $data = $this->getData(InvoiceInterface::COMMENTS);
+
+        if (!$data instanceof \Magento\Sales\Model\ResourceModel\Order\Invoice\Comment\Collection || $reload) {
             $comments = $this->_commentCollectionFactory->create()->setInvoiceFilter($this->getId())
                 ->setCreatedAtOrder();
 
             $this->setComments($comments);
             /**
-             * When invoice created with adding comment, comments collection
+             * When invoice created with adding comment, a comments collection
              * must be loaded before we added this comment.
              */
-            $this->getComments()->load();
+            $comments->load();
 
             if ($this->getId()) {
-                foreach ($this->getComments() as $comment) {
+                foreach ($comments as $comment) {
                     $comment->setInvoice($this);
                 }
             }
+
+            return $comments;
         }
-        return $this->getComments();
+        return $data;
     }
 
     /**
@@ -808,13 +812,20 @@ public function getItems()
      */
     public function getComments()
     {
-        if ($this->getData(InvoiceInterface::COMMENTS) === null && $this->getId()) {
+        $data = $this->getData(InvoiceInterface::COMMENTS);
+
+        if ($data instanceof \Magento\Sales\Model\ResourceModel\Order\Invoice\Comment\Collection) {
+            return $data->getItems();
+        }
+
+        if ($data === null && $this->getId()) {
             $collection = $this->_commentCollectionFactory->create()->setInvoiceFilter($this->getId());
             foreach ($collection as $comment) {
                 $comment->setInvoice($this);
             }
             $this->setData(InvoiceInterface::COMMENTS, $collection->getItems());
         }
+
         return $this->getData(InvoiceInterface::COMMENTS);
     }
 

app/code/Magento/Sales/Test/Unit/Model/Order/InvoiceTest.php

@@ -488,4 +488,83 @@ public static function getNotOpenedInvoiceStatuses()
             [Invoice::STATE_CANCELED, Invoice::STATE_CANCELED],
         ];
     }
+
+    /**
+     * Test that getComments() and getCommentsCollection() work together without type conflicts
+     *
+     * This test verifies the fix for the issue where calling getComments() before addComment()
+     * would cause a fatal error because getComments() stores an array while
+     * getCommentsCollection() expects a Collection object.
+     */
+    public function testCommentsAndCommentsCollectionTypeCompatibility()
+    {
+        $commentCollection = $this->getMockBuilder(
+            \Magento\Sales\Model\ResourceModel\Order\Invoice\Comment\Collection::class
+        )->disableOriginalConstructor()
+            ->onlyMethods(['setInvoiceFilter', 'setCreatedAtOrder', 'load', 'getItems', 'getIterator'])
+            ->getMock();
+        $commentCollection->expects($this->any())
+            ->method('setInvoiceFilter')
+            ->willReturnSelf();
+        $commentCollection->expects($this->any())
+            ->method('setCreatedAtOrder')
+            ->willReturnSelf();
+        $commentCollection->expects($this->any())
+            ->method('load')
+            ->willReturnSelf();
+        $commentCollection->expects($this->any())
+            ->method('getItems')
+            ->willReturn([]);
+        $commentCollection->expects($this->any())
+            ->method('getIterator')
+            ->willReturn(new \ArrayIterator([]));
+
+        $commentCollectionFactory = $this->createMock(CommentCollectionFactory::class);
+        $commentCollectionFactory->expects($this->any())
+            ->method('create')
+            ->willReturn($commentCollection);
+
+        $objectManager = new ObjectManager($this);
+        $invoice = $objectManager->getObject(
+            Invoice::class,
+            [
+                'commentCollectionFactory' => $commentCollectionFactory,
+            ]
+        );
+        $invoice->setId(1);
+
+        // Test scenario 1: getComments() returns array
+        $comments = $invoice->getComments();
+        $this->assertIsArray($comments, 'getComments() should return an array');
+
+        // Test scenario 2: getCommentsCollection() returns Collection even after getComments() stored array
+        $collection = $invoice->getCommentsCollection();
+        $this->assertInstanceOf(
+            \Magento\Sales\Model\ResourceModel\Order\Invoice\Comment\Collection::class,
+            $collection,
+            'getCommentsCollection() should return Collection instance even after getComments() was called'
+        );
+
+        // Test scenario 3: getComments() returns array even after getCommentsCollection() stored Collection
+        $invoice->getCommentsCollection();
+        $commentsAfterCollection = $invoice->getComments();
+        $this->assertIsArray(
+            $commentsAfterCollection,
+            'getComments() should always return array, even after getCommentsCollection() was called'
+        );
+
+        // Test scenario 4: Multiple calls maintain type consistency
+        $collection2 = $invoice->getCommentsCollection();
+        $this->assertInstanceOf(
+            \Magento\Sales\Model\ResourceModel\Order\Invoice\Comment\Collection::class,
+            $collection2,
+            'getCommentsCollection() should consistently return Collection instance'
+        );
+
+        $comments2 = $invoice->getComments();
+        $this->assertIsArray(
+            $comments2,
+            'getComments() should consistently return array'
+        );
+    }
 }