Fix the error that is wrong link title of a downloadable product when enabling "Use Default Value"

app/code/Magento/Downloadable/Model/Link/Builder.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Downloadable\Model\Link;
 
 use Magento\Downloadable\Helper\File;
@@ -12,7 +13,8 @@
 use Magento\Framework\DataObject\Copy;
 
 /**
- * Class Builder
+ * Builder download link model for downloadable product
+ *
  * @api
  * @since 100.1.0
  */
@@ -116,7 +118,7 @@ public function build(\Magento\Downloadable\Api\Data\LinkInterface $link)
             $link->setLinkFile($linkFileName);
             $link->setLinkUrl(null);
         }
-        
+
         if (isset($this->data['sample'])) {
             $link = $this->buildSample($link, $this->data['sample']);
         }
@@ -132,6 +134,12 @@ public function build(\Magento\Downloadable\Api\Data\LinkInterface $link)
         if (isset($this->data['is_unlimited']) && $this->data['is_unlimited']) {
             $link->setNumberOfDownloads(0);
         }
+
+        $useDefaultTitle = $this->data['use_default_title'] ?? false;
+
+        if ($useDefaultTitle) {
+            $link->setTitle(null);
+        }
         $this->resetData();
 
         return $link;

app/code/Magento/Downloadable/Model/Sample/Builder.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Downloadable\Model\Sample;
 
 use Magento\Downloadable\Api\Data\SampleInterface;
@@ -14,7 +15,8 @@
 use Magento\Framework\DataObject\Copy;
 
 /**
- * Class Builder
+ * Builder download sample link model for downloadable product
+ *
  * @api
  * @since 100.1.0
  */
@@ -24,7 +26,7 @@ class Builder
      * @var Sample
      */
     private $component;
-    
+
     /**
      * @var File
      */
@@ -71,6 +73,8 @@ public function __construct(
     }
 
     /**
+     * Init data for builder
+     *
      * @param array $data
      * @return $this;
      * @since 100.1.0
@@ -82,6 +86,8 @@ public function setData(array $data)
     }
 
     /**
+     * Build sample link
+     *
      * @param SampleInterface $sample
      * @return SampleInterface
      * @throws \Magento\Framework\Exception\LocalizedException
@@ -122,6 +128,8 @@ public function build(SampleInterface $sample)
     }
 
     /**
+     * Reset data
+     *
      * @return void
      */
     private function resetData()
@@ -130,6 +138,8 @@ private function resetData()
     }
 
     /**
+     * Get component
+     *
      * @return Sample
      */
     private function getComponent()

app/code/Magento/Downloadable/Test/Unit/Model/Link/BuilderTest.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Downloadable\Test\Unit\Model\Link;
 
 use Magento\Downloadable\Api\Data\LinkInterface;
@@ -11,7 +12,7 @@
 use Magento\Downloadable\Helper\Download;
 
 /**
- * Class BuilderTest
+ * Unit test for downloadable products' builder link class
  */
 class BuilderTest extends \PHPUnit\Framework\TestCase
 {
@@ -68,7 +69,7 @@ protected function setUp()
         $this->linkMock = $this->getMockBuilder(LinkInterface::class)
             ->disableOriginalConstructor()
             ->getMockForAbstractClass();
-        
+
         $this->service = $objectManagerHelper->getObject(
             Builder::class,
             [
@@ -160,6 +161,10 @@ public function testBuild($data, $expectedPrice)
         if (isset($data['is_unlimited'])) {
             $this->linkMock->expects($this->once())->method('setNumberOfDownloads')->with(0);
         }
+        $useDefaultTitle = $data['use_default_title'] ?? false;
+        if ($useDefaultTitle) {
+            $this->linkMock->expects($this->once())->method('setTitle')->with(null);
+        }
         if (isset($data['price'])) {
             $this->linkMock->expects($this->once())->method('getPrice')->willReturn($data['price']);
         } else {
@@ -219,6 +224,7 @@ public function buildProvider()
                 [
                     'file' => 'cXVlIHRhbA==',
                     'type' => 'file',
+                    'use_default_title' => '1',
                     'sample' => [
                         'file' => 'cXVlIHRhbA==',
                         'type' => 'file'

app/code/Magento/Downloadable/Test/Unit/Model/Sample/BuilderTest.php

@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Downloadable\Test\Unit\Model\Sample;
 
 use Magento\Downloadable\Api\Data\SampleInterface;
@@ -11,7 +12,7 @@
 use Magento\Downloadable\Model\Sample\Builder;
 
 /**
- * Class BuilderTest
+ * Unit test for downloadable products' builder sample class
  */
 class BuilderTest extends \PHPUnit\Framework\TestCase
 {
@@ -123,7 +124,7 @@ public function testBuild()
         $this->sampleMock->expects($this->once())->method('setSampleFile')->with($fileName);
         $this->sampleMock->expects($this->once())->method('setSortOrder')->with(1);
         $this->service->setData($data);
-        
+
         $this->service->build($this->sampleMock);
     }
 }