diff --git a/app/code/Magento/Catalog/Block/Product/ProductList/Related.php b/app/code/Magento/Catalog/Block/Product/ProductList/Related.php
index 24811d61a7715..387fac770c5bc 100644
--- a/app/code/Magento/Catalog/Block/Product/ProductList/Related.php
+++ b/app/code/Magento/Catalog/Block/Product/ProductList/Related.php
@@ -6,7 +6,15 @@
 
 namespace Magento\Catalog\Block\Product\ProductList;
 
+use Magento\Catalog\Block\Product\AbstractProduct;
+use Magento\Catalog\Block\Product\Context;
+use Magento\Catalog\Model\Product;
+use Magento\Catalog\Model\Product\Visibility as ProductVisibility;
 use Magento\Catalog\Model\ResourceModel\Product\Collection;
+use Magento\Checkout\Model\ResourceModel\Cart as CartResourceModel;
+use Magento\Checkout\Model\Session as CheckoutSession;
+use Magento\Framework\DataObject\IdentityInterface;
+use Magento\Framework\Module\Manager;
 use Magento\Framework\View\Element\AbstractBlock;
 
 /**
@@ -16,8 +24,7 @@
  * @SuppressWarnings(PHPMD.LongVariable)
  * @since 100.0.2
  */
-class Related extends \Magento\Catalog\Block\Product\AbstractProduct implements
-    \Magento\Framework\DataObject\IdentityInterface
+class Related extends AbstractProduct implements IdentityInterface
 {
     /**
      * @var Collection
@@ -27,53 +34,50 @@ class Related extends \Magento\Catalog\Block\Product\AbstractProduct implements
     /**
      * Checkout session
      *
-     * @var \Magento\Checkout\Model\Session
+     * @var CheckoutSession
      */
     protected $_checkoutSession;
 
     /**
      * Catalog product visibility
      *
-     * @var \Magento\Catalog\Model\Product\Visibility
+     * @var ProductVisibility
      */
     protected $_catalogProductVisibility;
 
     /**
      * Checkout cart
      *
-     * @var \Magento\Checkout\Model\ResourceModel\Cart
+     * @var CartResourceModel
      */
     protected $_checkoutCart;
 
     /**
-     * @var \Magento\Framework\Module\Manager
+     * @var Manager
      */
     protected $moduleManager;
 
     /**
-     * @param \Magento\Catalog\Block\Product\Context $context
-     * @param \Magento\Checkout\Model\ResourceModel\Cart $checkoutCart
-     * @param \Magento\Catalog\Model\Product\Visibility $catalogProductVisibility
-     * @param \Magento\Checkout\Model\Session $checkoutSession
-     * @param \Magento\Framework\Module\Manager $moduleManager
+     * @param Context $context
+     * @param CartResourceModel $checkoutCart
+     * @param ProductVisibility $catalogProductVisibility
+     * @param CheckoutSession $checkoutSession
+     * @param Manager $moduleManager
      * @param array $data
      */
     public function __construct(
-        \Magento\Catalog\Block\Product\Context $context,
-        \Magento\Checkout\Model\ResourceModel\Cart $checkoutCart,
-        \Magento\Catalog\Model\Product\Visibility $catalogProductVisibility,
-        \Magento\Checkout\Model\Session $checkoutSession,
-        \Magento\Framework\Module\Manager $moduleManager,
+        Context $context,
+        CartResourceModel $checkoutCart,
+        ProductVisibility $catalogProductVisibility,
+        CheckoutSession $checkoutSession,
+        Manager $moduleManager,
         array $data = []
     ) {
         $this->_checkoutCart = $checkoutCart;
         $this->_catalogProductVisibility = $catalogProductVisibility;
         $this->_checkoutSession = $checkoutSession;
         $this->moduleManager = $moduleManager;
-        parent::__construct(
-            $context,
-            $data
-        );
+        parent::__construct($context, $data);
     }
 
     /**
@@ -84,7 +88,7 @@ public function __construct(
     protected function _prepareData()
     {
         $product = $this->getProduct();
-        /* @var $product \Magento\Catalog\Model\Product */
+        /* @var $product Product */
 
         $this->_itemCollection = $product->getRelatedProductCollection()->addAttributeToSelect(
             'required_options'
@@ -139,12 +143,11 @@ public function getItems()
      */
     public function getIdentities()
     {
-        $identities = [];
+        $identities = [[]];
         foreach ($this->getItems() as $item) {
-            // phpcs:ignore Magento2.Performance.ForeachArrayMerge
-            $identities = array_merge($identities, $item->getIdentities());
+            $identities[] = $item->getIdentities();
         }
-        return $identities;
+        return array_merge(...$identities);
     }
 
     /**
diff --git a/app/code/Magento/Catalog/Block/Product/ProductList/Upsell.php b/app/code/Magento/Catalog/Block/Product/ProductList/Upsell.php
index fa1beaf6e0ea8..ac66392efe5dc 100644
--- a/app/code/Magento/Catalog/Block/Product/ProductList/Upsell.php
+++ b/app/code/Magento/Catalog/Block/Product/ProductList/Upsell.php
@@ -6,7 +6,16 @@
 
 namespace Magento\Catalog\Block\Product\ProductList;
 
+use Magento\Catalog\Block\Product\AbstractProduct;
+use Magento\Catalog\Block\Product\Context;
+use Magento\Catalog\Model\Product;
+use Magento\Catalog\Model\Product\Visibility as ProductVisibility;
 use Magento\Catalog\Model\ResourceModel\Product\Collection;
+use Magento\Checkout\Model\ResourceModel\Cart as CartResourceModel;
+use Magento\Checkout\Model\Session as CheckoutSession;
+use Magento\Framework\DataObject;
+use Magento\Framework\DataObject\IdentityInterface;
+use Magento\Framework\Module\Manager;
 
 /**
  * Catalog product upsell items block
@@ -15,8 +24,7 @@
  * @SuppressWarnings(PHPMD.LongVariable)
  * @since 100.0.2
  */
-class Upsell extends \Magento\Catalog\Block\Product\AbstractProduct implements
-    \Magento\Framework\DataObject\IdentityInterface
+class Upsell extends AbstractProduct implements IdentityInterface
 {
     /**
      * @var int
@@ -24,7 +32,7 @@ class Upsell extends \Magento\Catalog\Block\Product\AbstractProduct implements
     protected $_columnCount = 4;
 
     /**
-     * @var  \Magento\Framework\DataObject[]
+     * @var  DataObject[]
      */
     protected $_items;
 
@@ -41,53 +49,50 @@ class Upsell extends \Magento\Catalog\Block\Product\AbstractProduct implements
     /**
      * Checkout session
      *
-     * @var \Magento\Checkout\Model\Session
+     * @var CheckoutSession
      */
     protected $_checkoutSession;
 
     /**
      * Catalog product visibility
      *
-     * @var \Magento\Catalog\Model\Product\Visibility
+     * @var ProductVisibility
      */
     protected $_catalogProductVisibility;
 
     /**
      * Checkout cart
      *
-     * @var \Magento\Checkout\Model\ResourceModel\Cart
+     * @var CartResourceModel
      */
     protected $_checkoutCart;
 
     /**
-     * @var \Magento\Framework\Module\Manager
+     * @var Manager
      */
     protected $moduleManager;
 
     /**
-     * @param \Magento\Catalog\Block\Product\Context $context
-     * @param \Magento\Checkout\Model\ResourceModel\Cart $checkoutCart
-     * @param \Magento\Catalog\Model\Product\Visibility $catalogProductVisibility
-     * @param \Magento\Checkout\Model\Session $checkoutSession
-     * @param \Magento\Framework\Module\Manager $moduleManager
+     * @param Context $context
+     * @param CartResourceModel $checkoutCart
+     * @param ProductVisibility $catalogProductVisibility
+     * @param CheckoutSession $checkoutSession
+     * @param Manager $moduleManager
      * @param array $data
      */
     public function __construct(
-        \Magento\Catalog\Block\Product\Context $context,
-        \Magento\Checkout\Model\ResourceModel\Cart $checkoutCart,
-        \Magento\Catalog\Model\Product\Visibility $catalogProductVisibility,
-        \Magento\Checkout\Model\Session $checkoutSession,
-        \Magento\Framework\Module\Manager $moduleManager,
+        Context $context,
+        CartResourceModel $checkoutCart,
+        ProductVisibility $catalogProductVisibility,
+        CheckoutSession $checkoutSession,
+        Manager $moduleManager,
         array $data = []
     ) {
         $this->_checkoutCart = $checkoutCart;
         $this->_catalogProductVisibility = $catalogProductVisibility;
         $this->_checkoutSession = $checkoutSession;
         $this->moduleManager = $moduleManager;
-        parent::__construct(
-            $context,
-            $data
-        );
+        parent::__construct($context, $data);
     }
 
     /**
@@ -98,7 +103,7 @@ public function __construct(
     protected function _prepareData()
     {
         $product = $this->getProduct();
-        /* @var $product \Magento\Catalog\Model\Product */
+        /* @var $product Product */
         $this->_itemCollection = $product->getUpSellProductCollection()->setPositionOrder()->addStoreFilter();
         if ($this->moduleManager->isEnabled('Magento_Checkout')) {
             $this->_addProductAttributesAndPrices($this->_itemCollection);
@@ -181,8 +186,8 @@ public function getRowCount()
      */
     public function setColumnCount($columns)
     {
-        if ((int) $columns > 0) {
-            $this->_columnCount = (int) $columns;
+        if ((int)$columns > 0) {
+            $this->_columnCount = (int)$columns;
         }
         return $this;
     }
@@ -231,8 +236,8 @@ public function getIterableItem()
      */
     public function setItemLimit($type, $limit)
     {
-        if ((int) $limit > 0) {
-            $this->_itemLimits[$type] = (int) $limit;
+        if ((int)$limit > 0) {
+            $this->_itemLimits[$type] = (int)$limit;
         }
         return $this;
     }
@@ -250,9 +255,9 @@ public function getItemLimit($type = '')
         }
         if (isset($this->_itemLimits[$type])) {
             return $this->_itemLimits[$type];
-        } else {
-            return 0;
         }
+
+        return 0;
     }
 
     /**
@@ -262,11 +267,10 @@ public function getItemLimit($type = '')
      */
     public function getIdentities()
     {
-        $identities = [];
-        foreach ($this->getItems() as $item) {
-            // phpcs:ignore Magento2.Performance.ForeachArrayMerge
-            $identities = array_merge($identities, $item->getIdentities());
-        }
-        return $identities;
+        $identities = array_map(function (DataObject $item) {
+            return $item->getIdentities();
+        }, $this->getItems()) ?: [[]];
+
+        return array_merge(...$identities);
     }
 }
diff --git a/app/code/Magento/Catalog/Test/Unit/Block/Product/ProductList/UpsellTest.php b/app/code/Magento/Catalog/Test/Unit/Block/Product/ProductList/UpsellTest.php
index 03f3c83d87dcb..f3aa3f182d198 100644
--- a/app/code/Magento/Catalog/Test/Unit/Block/Product/ProductList/UpsellTest.php
+++ b/app/code/Magento/Catalog/Test/Unit/Block/Product/ProductList/UpsellTest.php
@@ -5,17 +5,21 @@
  */
 namespace Magento\Catalog\Test\Unit\Block\Product\ProductList;
 
+use Magento\Catalog\Block\Product\ProductList\Upsell as UpsellBlock;
+use Magento\Catalog\Model\Product;
+
 class UpsellTest extends \PHPUnit\Framework\TestCase
 {
+    const STUB_EMPTY_ARRAY = [];
     /**
-     * @var \Magento\Catalog\Block\Product\ProductList\Upsell
+     * @var UpsellBlock
      */
     protected $block;
 
     protected function setUp()
     {
         $objectManager = new \Magento\Framework\TestFramework\Unit\Helper\ObjectManager($this);
-        $this->block = $objectManager->getObject(\Magento\Catalog\Block\Product\ProductList\Upsell::class);
+        $this->block = $objectManager->getObject(UpsellBlock::class);
     }
 
     protected function tearDown()
@@ -26,10 +30,10 @@ protected function tearDown()
     public function testGetIdentities()
     {
         $productTag = ['compare_item_1'];
-        $product = $this->createMock(\Magento\Catalog\Model\Product::class);
+        $product = $this->createMock(Product::class);
         $product->expects($this->once())->method('getIdentities')->will($this->returnValue($productTag));
 
-        $itemsCollection = new \ReflectionProperty(\Magento\Catalog\Block\Product\ProductList\Upsell::class, '_items');
+        $itemsCollection = new \ReflectionProperty(UpsellBlock::class, '_items');
         $itemsCollection->setAccessible(true);
         $itemsCollection->setValue($this->block, [$product]);
 
@@ -38,4 +42,32 @@ public function testGetIdentities()
             $this->block->getIdentities()
         );
     }
+
+    public function testGetIdentitiesWhenItemGetIdentitiesReturnEmptyArray()
+    {
+        $product = $this->createMock(Product::class);
+        $product->expects($this->once())->method('getIdentities')
+            ->willReturn(self::STUB_EMPTY_ARRAY);
+
+        $itemsCollection = new \ReflectionProperty(UpsellBlock::class, '_items');
+        $itemsCollection->setAccessible(true);
+        $itemsCollection->setValue($this->block, [$product]);
+
+        $this->assertEquals(
+            self::STUB_EMPTY_ARRAY,
+            $this->block->getIdentities()
+        );
+    }
+
+    public function testGetIdentitiesWhenGetItemsReturnEmptyArray()
+    {
+        $itemsCollection = new \ReflectionProperty(UpsellBlock::class, '_items');
+        $itemsCollection->setAccessible(true);
+        $itemsCollection->setValue($this->block, self::STUB_EMPTY_ARRAY);
+
+        $this->assertEquals(
+            self::STUB_EMPTY_ARRAY,
+            $this->block->getIdentities()
+        );
+    }
 }
diff --git a/app/code/Magento/CatalogInventory/Model/StockIndex.php b/app/code/Magento/CatalogInventory/Model/StockIndex.php
index 41db057cf149c..ad0cff43c6ac9 100644
--- a/app/code/Magento/CatalogInventory/Model/StockIndex.php
+++ b/app/code/Magento/CatalogInventory/Model/StockIndex.php
@@ -6,16 +6,20 @@
 
 namespace Magento\CatalogInventory\Model;
 
+use Magento\Catalog\Model\Product;
 use Magento\Catalog\Model\Product\Type as ProductType;
 use Magento\Catalog\Model\Product\Website as ProductWebsite;
-use Magento\Catalog\Model\ProductFactory;
 use Magento\CatalogInventory\Api\StockIndexInterface;
+use Magento\CatalogInventory\Model\ResourceModel\Stock\Status as StockStatusResourceModel;
 use Magento\CatalogInventory\Model\Spi\StockRegistryProviderInterface;
 use Magento\Catalog\Api\ProductRepositoryInterface;
 use Magento\Catalog\Model\Product\Attribute\Source\Status;
+use Magento\Framework\App\ObjectManager;
+use Magento\Store\Model\Website;
 
 /**
- * Class StockIndex
+ * Index responsible for Stock
+ *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
 class StockIndex implements StockIndexInterface
@@ -31,7 +35,7 @@ class StockIndex implements StockIndexInterface
     protected $productRepository;
 
     /**
-     * @var \Magento\CatalogInventory\Model\ResourceModel\Stock\Status
+     * @var StockStatusResourceModel
      */
     protected $stockStatusResource;
 
@@ -47,6 +51,11 @@ class StockIndex implements StockIndexInterface
      */
     protected $websites;
 
+    /**
+     * @var ProductWebsite
+     */
+    private $productWebsite;
+
     /**
      * Product Type Instances cache
      *
@@ -88,7 +97,7 @@ public function rebuild($productId = null, $scopeId = null)
         } else {
             $lastProductId = 0;
             while (true) {
-                /** @var \Magento\CatalogInventory\Model\ResourceModel\Stock\Status $resource */
+                /** @var StockStatusResourceModel $resource */
                 $resource = $this->getStockStatusResource();
                 $productCollection = $resource->getProductCollection($lastProductId);
                 if (!$productCollection) {
@@ -115,7 +124,7 @@ public function updateProductStockStatus($productId, $websiteId)
     {
         $item = $this->stockRegistryProvider->getStockItem($productId, $websiteId);
 
-        $status = \Magento\CatalogInventory\Model\Stock\Status::STATUS_IN_STOCK;
+        $status = Stock\Status::STATUS_IN_STOCK;
         $qty = 0;
         if ($item->getItemId()) {
             $status = $item->getIsInStock();
@@ -132,16 +141,16 @@ public function updateProductStockStatus($productId, $websiteId)
      * @param int $websiteId
      * @param int $qty
      * @param int $status
-     * @return $this
+     * @return void
      * @SuppressWarnings(PHPMD.CyclomaticComplexity)
      */
     protected function processChildren(
         $productId,
         $websiteId,
         $qty = 0,
-        $status = \Magento\CatalogInventory\Model\Stock\Status::STATUS_IN_STOCK
+        $status = Stock\Status::STATUS_IN_STOCK
     ) {
-        if ($status == \Magento\CatalogInventory\Model\Stock\Status::STATUS_OUT_OF_STOCK) {
+        if ($status == Stock\Status::STATUS_OUT_OF_STOCK) {
             $this->getStockStatusResource()->saveProductStatus($productId, $status, $qty, $websiteId);
             return;
         }
@@ -150,20 +159,22 @@ protected function processChildren(
         $websitesWithStores = $this->getWebsitesWithDefaultStores($websiteId);
 
         foreach (array_keys($websitesWithStores) as $websiteId) {
-            /* @var $website \Magento\Store\Model\Website */
+            /* @var $website Website */
             $statuses[$websiteId] = $status;
         }
 
-        /** @var \Magento\Catalog\Model\Product $product */
+        /** @var Product $product */
         $product = $this->productRepository->getById($productId);
         $typeInstance = $product->getTypeInstance();
 
         $requiredChildrenIds = $typeInstance->getChildrenIds($productId, true);
         if ($requiredChildrenIds) {
-            $childrenIds = [];
+            $childrenIds = [[]];
             foreach ($requiredChildrenIds as $groupedChildrenIds) {
-                $childrenIds = array_merge($childrenIds, $groupedChildrenIds);
+                $childrenIds[] = $groupedChildrenIds;
             }
+            $childrenIds = array_merge(...$childrenIds);
+
             $childrenWebsites = $this->productWebsite->getWebsites($childrenIds);
             foreach ($websitesWithStores as $websiteId => $storeId) {
                 $childrenStatus = $this->getStockStatusResource()->getProductStatus($childrenIds, $storeId);
@@ -177,7 +188,7 @@ protected function processChildren(
                             && in_array($websiteId, $childrenWebsites[$childId])
                             && $childrenStatus[$childId] == Status::STATUS_ENABLED
                             && isset($childrenStock[$childId])
-                            && $childrenStock[$childId] == \Magento\CatalogInventory\Model\Stock\Status::STATUS_IN_STOCK
+                            && $childrenStock[$childId] == Stock\Status::STATUS_IN_STOCK
                         ) {
                             $optionStatus = true;
                         }
@@ -196,12 +207,12 @@ protected function processChildren(
      * Retrieve website models
      *
      * @param int|null $websiteId
-     * @return array
+     * @return Website[]
      */
     protected function getWebsitesWithDefaultStores($websiteId = null)
     {
         if ($this->websites === null) {
-            /** @var \Magento\CatalogInventory\Model\ResourceModel\Stock\Status $resource */
+            /** @var StockStatusResourceModel $resource */
             $resource = $this->getStockStatusResource();
             $this->websites = $resource->getWebsiteStores();
         }
@@ -217,23 +228,25 @@ protected function getWebsitesWithDefaultStores($websiteId = null)
      *
      * @param int $productId
      * @param int $websiteId
-     * @return $this
+     * @return void
      */
     protected function processParents($productId, $websiteId)
     {
-        $parentIds = [];
+        $parentIds = [[]];
         foreach ($this->getProductTypeInstances() as $typeInstance) {
-            /* @var $typeInstance AbstractType */
-            $parentIds = array_merge($parentIds, $typeInstance->getParentIdsByChild($productId));
+            /* @var ProductType\AbstractType $typeInstance */
+            $parentIds[] = $typeInstance->getParentIdsByChild($productId);
         }
 
-        if (!$parentIds) {
-            return $this;
+        $parentIds = array_merge(...$parentIds);
+
+        if (empty($parentIds)) {
+            return;
         }
 
         foreach ($parentIds as $parentId) {
             $item = $this->stockRegistryProvider->getStockItem($parentId, $websiteId);
-            $status = \Magento\CatalogInventory\Model\Stock\Status::STATUS_IN_STOCK;
+            $status = Stock\Status::STATUS_IN_STOCK;
             $qty = 0;
             if ($item->getItemId()) {
                 $status = $item->getIsInStock();
@@ -244,10 +257,9 @@ protected function processParents($productId, $websiteId)
     }
 
     /**
-     * Retrieve Product Type Instances
-     * as key - type code, value - instance model
+     * Retrieve Product Type Instances as key - type code, value - instance model
      *
-     * @return array
+     * @return ProductType\AbstractType[]
      */
     protected function getProductTypeInstances()
     {
@@ -262,14 +274,14 @@ protected function getProductTypeInstances()
     }
 
     /**
-     * @return \Magento\CatalogInventory\Model\ResourceModel\Stock\Status
+     * Returns ResourceModel for Stock Status
+     *
+     * @return StockStatusResourceModel
      */
     protected function getStockStatusResource()
     {
         if (empty($this->stockStatusResource)) {
-            $this->stockStatusResource = \Magento\Framework\App\ObjectManager::getInstance()->get(
-                \Magento\CatalogInventory\Model\ResourceModel\Stock\Status::class
-            );
+            $this->stockStatusResource = ObjectManager::getInstance()->get(StockStatusResourceModel::class);
         }
         return $this->stockStatusResource;
     }
diff --git a/app/code/Magento/CatalogSearch/Model/Search/FiltersExtractor.php b/app/code/Magento/CatalogSearch/Model/Search/FiltersExtractor.php
index 55c8582979912..63d7ee0e6e4c0 100644
--- a/app/code/Magento/CatalogSearch/Model/Search/FiltersExtractor.php
+++ b/app/code/Magento/CatalogSearch/Model/Search/FiltersExtractor.php
@@ -10,11 +10,10 @@
 use Magento\Framework\Search\Request\Filter\BoolExpression;
 
 /**
- * Class FiltersExtractor
  * Extracts filters from QueryInterface
  *
- * @deprecated
- * @see \Magento\ElasticSearch
+ * @deprecated Use Magento\Elasticsearch implementation of QueryInterface
+ * @see \Magento\Elasticsearch
  */
 class FiltersExtractor
 {
@@ -26,19 +25,19 @@ class FiltersExtractor
      */
     public function extractFiltersFromQuery(QueryInterface $query)
     {
-        $filters = [];
+        $filters = [[]];
 
         switch ($query->getType()) {
             case QueryInterface::TYPE_BOOL:
                 /** @var \Magento\Framework\Search\Request\Query\BoolExpression $query */
                 foreach ($query->getMust() as $subQuery) {
-                    $filters = array_merge($filters, $this->extractFiltersFromQuery($subQuery));
+                    $filters[] = $this->extractFiltersFromQuery($subQuery);
                 }
                 foreach ($query->getShould() as $subQuery) {
-                    $filters = array_merge($filters, $this->extractFiltersFromQuery($subQuery));
+                    $filters[] = $this->extractFiltersFromQuery($subQuery);
                 }
                 foreach ($query->getMustNot() as $subQuery) {
-                    $filters = array_merge($filters, $this->extractFiltersFromQuery($subQuery));
+                    $filters[] = $this->extractFiltersFromQuery($subQuery);
                 }
                 break;
 
@@ -46,9 +45,9 @@ public function extractFiltersFromQuery(QueryInterface $query)
                 /** @var Filter $query */
                 $filter = $query->getReference();
                 if (FilterInterface::TYPE_BOOL === $filter->getType()) {
-                    $filters = array_merge($filters, $this->getFiltersFromBoolFilter($filter));
+                    $filters[] = $this->getFiltersFromBoolFilter($filter);
                 } else {
-                    $filters[] = $filter;
+                    $filters[] = [$filter];
                 }
                 break;
 
@@ -56,39 +55,41 @@ public function extractFiltersFromQuery(QueryInterface $query)
                 break;
         }
 
-        return $filters;
+        return array_merge(...$filters);
     }
 
     /**
+     * Returns list of filters from Bool filter
+     *
      * @param BoolExpression $boolExpression
      * @return FilterInterface[]
      */
     private function getFiltersFromBoolFilter(BoolExpression $boolExpression)
     {
-        $filters = [];
+        $filters = [[]];
 
         /** @var BoolExpression $filter */
         foreach ($boolExpression->getMust() as $filter) {
             if ($filter->getType() === FilterInterface::TYPE_BOOL) {
-                $filters = array_merge($filters, $this->getFiltersFromBoolFilter($filter));
+                $filters[] = $this->getFiltersFromBoolFilter($filter);
             } else {
-                $filters[] = $filter;
+                $filters[] = [$filter];
             }
         }
         foreach ($boolExpression->getShould() as $filter) {
             if ($filter->getType() === FilterInterface::TYPE_BOOL) {
-                $filters = array_merge($filters, $this->getFiltersFromBoolFilter($filter));
+                $filters[] = $this->getFiltersFromBoolFilter($filter);
             } else {
-                $filters[] = $filter;
+                $filters[] = [$filter];
             }
         }
         foreach ($boolExpression->getMustNot() as $filter) {
             if ($filter->getType() === FilterInterface::TYPE_BOOL) {
-                $filters = array_merge($filters, $this->getFiltersFromBoolFilter($filter));
+                $filters[] = $this->getFiltersFromBoolFilter($filter);
             } else {
-                $filters[] = $filter;
+                $filters[] = [$filter];
             }
         }
-        return $filters;
+        return array_merge(...$filters);
     }
 }
diff --git a/app/code/Magento/CatalogUrlRewrite/Model/Category/Plugin/Store/Group.php b/app/code/Magento/CatalogUrlRewrite/Model/Category/Plugin/Store/Group.php
index 1dbfad6cf5219..308b82e38c43a 100644
--- a/app/code/Magento/CatalogUrlRewrite/Model/Category/Plugin/Store/Group.php
+++ b/app/code/Magento/CatalogUrlRewrite/Model/Category/Plugin/Store/Group.php
@@ -5,6 +5,7 @@
  */
 namespace Magento\CatalogUrlRewrite\Model\Category\Plugin\Store;
 
+use Magento\Store\Model\ResourceModel\Group as StoreGroup;
 use Magento\UrlRewrite\Model\UrlPersistInterface;
 use Magento\UrlRewrite\Service\V1\Data\UrlRewrite;
 use Magento\Catalog\Model\CategoryFactory;
@@ -16,37 +17,39 @@
 use Magento\Framework\Model\AbstractModel;
 
 /**
+ * Generate Product and Category URLs
+ *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
 class Group
 {
     /**
-     * @var \Magento\UrlRewrite\Model\UrlPersistInterface
+     * @var UrlPersistInterface
      */
     protected $urlPersist;
 
     /**
-     * @var \Magento\Catalog\Model\CategoryFactory
+     * @var CategoryFactory
      */
     protected $categoryFactory;
 
     /**
-     * @var \Magento\Catalog\Model\ProductFactory
+     * @var ProductFactory
      */
     protected $productFactory;
 
     /**
-     * @var \Magento\CatalogUrlRewrite\Model\CategoryUrlRewriteGenerator
+     * @var CategoryUrlRewriteGenerator
      */
     protected $categoryUrlRewriteGenerator;
 
     /**
-     * @var \Magento\CatalogUrlRewrite\Model\ProductUrlRewriteGenerator
+     * @var ProductUrlRewriteGenerator
      */
     protected $productUrlRewriteGenerator;
 
     /**
-     * @var \Magento\Store\Model\StoreManagerInterface
+     * @var StoreManagerInterface
      */
     protected $storeManager;
 
@@ -77,15 +80,15 @@ public function __construct(
     /**
      * Perform updating url for categories and products assigned to the group
      *
-     * @param \Magento\Store\Model\ResourceModel\Group $subject
-     * @param \Magento\Store\Model\ResourceModel\Group $result
+     * @param StoreGroup $subject
+     * @param StoreGroup $result
      * @param AbstractModel $group
-     * @return \Magento\Store\Model\ResourceModel\Group
+     * @return StoreGroup
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
     public function afterSave(
-        \Magento\Store\Model\ResourceModel\Group $subject,
-        \Magento\Store\Model\ResourceModel\Group $result,
+        StoreGroup $subject,
+        StoreGroup $result,
         AbstractModel $group
     ) {
         if (!$group->isObjectNew()
@@ -118,7 +121,7 @@ public function afterSave(
      */
     protected function generateProductUrls($websiteId, $originWebsiteId)
     {
-        $urls = [];
+        $urls = [[]];
         $websiteIds = $websiteId != $originWebsiteId
             ? [$websiteId, $originWebsiteId]
             : [$websiteId];
@@ -130,33 +133,30 @@ protected function generateProductUrls($websiteId, $originWebsiteId)
         foreach ($collection as $product) {
             /** @var \Magento\Catalog\Model\Product $product */
             $product->setStoreId(Store::DEFAULT_STORE_ID);
-            $urls = array_merge(
-                $urls,
-                $this->productUrlRewriteGenerator->generate($product)
-            );
+            $urls[] = $this->productUrlRewriteGenerator->generate($product);
         }
 
-        return $urls;
+        return array_merge(...$urls);
     }
 
     /**
+     * Generate url rewrites for categories assigned to store
+     *
      * @param int $rootCategoryId
      * @param array $storeIds
      * @return array
      */
     protected function generateCategoryUrls($rootCategoryId, $storeIds)
     {
-        $urls = [];
+        $urls = [[]];
         $categories = $this->categoryFactory->create()->getCategories($rootCategoryId, 1, false, true);
         foreach ($categories as $category) {
             /** @var \Magento\Catalog\Model\Category $category */
             $category->setStoreId(Store::DEFAULT_STORE_ID);
             $category->setStoreIds($storeIds);
-            $urls = array_merge(
-                $urls,
-                $this->categoryUrlRewriteGenerator->generate($category)
-            );
+            $urls[] = $this->categoryUrlRewriteGenerator->generate($category);
         }
-        return $urls;
+
+        return array_merge(...$urls);
     }
 }
diff --git a/app/code/Magento/CatalogWidget/Block/Product/ProductsList.php b/app/code/Magento/CatalogWidget/Block/Product/ProductsList.php
index 4a75c806aa37b..82fae8fc3875d 100644
--- a/app/code/Magento/CatalogWidget/Block/Product/ProductsList.php
+++ b/app/code/Magento/CatalogWidget/Block/Product/ProductsList.php
@@ -125,7 +125,7 @@ class ProductsList extends AbstractProduct implements BlockInterface, IdentityIn
     private $urlEncoder;
 
     /**
-     * @var \Magento\Framework\View\Element\RendererList
+     * @var RendererList
      */
     private $rendererListBlock;
 
@@ -194,14 +194,12 @@ protected function _construct()
             ->addColumnCountLayoutDepend('2columns-right', 4)
             ->addColumnCountLayoutDepend('3columns', 3);
 
-        $this->addData(
-            [
-                'cache_lifetime' => 86400,
-                'cache_tags' => [
-                    Product::CACHE_TAG,
-                ],
-            ]
-        );
+        $this->addData([
+            'cache_lifetime' => 86400,
+            'cache_tags' => [
+                Product::CACHE_TAG,
+            ],
+        ]);
     }
 
     /**
@@ -506,14 +504,15 @@ public function getPagerHtml()
      */
     public function getIdentities()
     {
-        $identities = [];
+        $identities = [[]];
         if ($this->getProductCollection()) {
             foreach ($this->getProductCollection() as $product) {
                 if ($product instanceof IdentityInterface) {
-                    $identities += $product->getIdentities();
+                    $identities[] = $product->getIdentities();
                 }
             }
         }
+        $identities = array_merge(...$identities);
 
         return $identities ?: [Product::CACHE_TAG];
     }
diff --git a/app/code/Magento/CheckoutAgreements/Model/AgreementsValidator.php b/app/code/Magento/CheckoutAgreements/Model/AgreementsValidator.php
index 8d25ba88c0f82..2643e69ba1efd 100644
--- a/app/code/Magento/CheckoutAgreements/Model/AgreementsValidator.php
+++ b/app/code/Magento/CheckoutAgreements/Model/AgreementsValidator.php
@@ -5,13 +5,15 @@
  */
 namespace Magento\CheckoutAgreements\Model;
 
+use Magento\Checkout\Api\AgreementsValidatorInterface;
+
 /**
- * Class AgreementsValidator
+ * Validator for Checkout Agreements
  */
-class AgreementsValidator implements \Magento\Checkout\Api\AgreementsValidatorInterface
+class AgreementsValidator implements AgreementsValidatorInterface
 {
     /**
-     * @var \Magento\CheckoutAgreements\Model\AgreementsProviderInterface[]
+     * @var AgreementsProviderInterface[]
      */
     protected $agreementsProviders;
 
@@ -33,11 +35,13 @@ public function __construct($list = null)
     public function isValid($agreementIds = [])
     {
         $agreementIds = $agreementIds === null ? [] : $agreementIds;
-        $requiredAgreements = [];
+        $requiredAgreements = [[]];
         foreach ($this->agreementsProviders as $agreementsProvider) {
-            $requiredAgreements = array_merge($requiredAgreements, $agreementsProvider->getRequiredAgreementIds());
+            $requiredAgreements[] = $agreementsProvider->getRequiredAgreementIds();
         }
-        $agreementsDiff = array_diff($requiredAgreements, $agreementIds);
+
+        $agreementsDiff = array_diff(array_merge(...$requiredAgreements), $agreementIds);
+
         return empty($agreementsDiff);
     }
 }
diff --git a/app/code/Magento/CurrencySymbol/Model/System/Currencysymbol.php b/app/code/Magento/CurrencySymbol/Model/System/Currencysymbol.php
index 6c7019986cce0..d48df02d9de27 100644
--- a/app/code/Magento/CurrencySymbol/Model/System/Currencysymbol.php
+++ b/app/code/Magento/CurrencySymbol/Model/System/Currencysymbol.php
@@ -292,7 +292,8 @@ protected function _unserializeStoreConfig($configPath, $storeId = null)
      */
     protected function getAllowedCurrencies()
     {
-        $allowedCurrencies = explode(
+        $allowedCurrencies = [[]];
+        $allowedCurrencies[] = explode(
             self::ALLOWED_CURRENCIES_CONFIG_SEPARATOR,
             $this->_scopeConfig->getValue(
                 self::XML_PATH_ALLOWED_CURRENCIES,
@@ -318,23 +319,17 @@ protected function getAllowedCurrencies()
                     if (!$websiteShow) {
                         $websiteShow = true;
                         $websiteSymbols = $website->getConfig(self::XML_PATH_ALLOWED_CURRENCIES);
-                        $allowedCurrencies = array_merge(
-                            $allowedCurrencies,
-                            explode(self::ALLOWED_CURRENCIES_CONFIG_SEPARATOR, $websiteSymbols)
-                        );
+                        $allowedCurrencies[] = explode(self::ALLOWED_CURRENCIES_CONFIG_SEPARATOR, $websiteSymbols);
                     }
                     $storeSymbols = $this->_scopeConfig->getValue(
                         self::XML_PATH_ALLOWED_CURRENCIES,
                         \Magento\Store\Model\ScopeInterface::SCOPE_STORE,
                         $store
                     );
-                    $allowedCurrencies = array_merge(
-                        $allowedCurrencies,
-                        explode(self::ALLOWED_CURRENCIES_CONFIG_SEPARATOR, $storeSymbols)
-                    );
+                    $allowedCurrencies[] = explode(self::ALLOWED_CURRENCIES_CONFIG_SEPARATOR, $storeSymbols);
                 }
             }
         }
-        return array_unique($allowedCurrencies);
+        return array_unique(array_merge(...$allowedCurrencies));
     }
 }
diff --git a/app/code/Magento/CurrencySymbol/Test/Unit/Observer/CurrencyDisplayOptionsTest.php b/app/code/Magento/CurrencySymbol/Test/Unit/Observer/CurrencyDisplayOptionsTest.php
index dd0d7ffc8cc68..ae8c12c2019b8 100644
--- a/app/code/Magento/CurrencySymbol/Test/Unit/Observer/CurrencyDisplayOptionsTest.php
+++ b/app/code/Magento/CurrencySymbol/Test/Unit/Observer/CurrencyDisplayOptionsTest.php
@@ -6,6 +6,7 @@
 namespace Magento\CurrencySymbol\Test\Unit\Observer;
 
 use Magento\CurrencySymbol\Model\System\CurrencysymbolFactory;
+use Magento\Framework\Locale\Currency;
 
 /**
  * Test for \Magento\CurrencySymbol\Observer\CurrencyDisplayOptions
@@ -65,7 +66,7 @@ protected function setUp()
     public function testCurrencyDisplayOptionsEmpty()
     {
         $baseData = [
-            \Magento\Framework\Locale\Currency::CURRENCY_OPTION_NAME => 'US Dollar'
+            Currency::CURRENCY_OPTION_NAME => 'US Dollar'
         ];
         $sampleCurrencyOptionObject = new \Magento\Framework\DataObject($baseData);
 
@@ -83,7 +84,7 @@ public function testCurrencyDisplayOptionsEmpty()
     public function testCurrencyDisplayOptions()
     {
         $baseData = [
-            \Magento\Framework\Locale\Currency::CURRENCY_OPTION_NAME => 'US Dollar'
+            Currency::CURRENCY_OPTION_NAME => 'US Dollar'
         ];
         $sampleCurrencyOptionObject = new \Magento\Framework\DataObject($baseData);
         $sampleCurrency = 'USD';
@@ -92,9 +93,9 @@ public function testCurrencyDisplayOptions()
         $expectedCurrencyOptions = array_merge(
             $baseData,
             [
-                \Magento\Framework\Locale\Currency::CURRENCY_OPTION_NAME => 'US Dollar',
-                \Magento\Framework\Locale\Currency::CURRENCY_OPTION_SYMBOL => $sampleCurrencySymbol,
-                \Magento\Framework\Locale\Currency::CURRENCY_OPTION_DISPLAY => \Magento\Framework\Currency::USE_SYMBOL
+                Currency::CURRENCY_OPTION_NAME => 'US Dollar',
+                Currency::CURRENCY_OPTION_SYMBOL => $sampleCurrencySymbol,
+                Currency::CURRENCY_OPTION_DISPLAY => \Magento\Framework\Currency::USE_SYMBOL
             ]
         );
 
diff --git a/app/code/Magento/Customer/Model/Address/CompositeValidator.php b/app/code/Magento/Customer/Model/Address/CompositeValidator.php
index 1d16a929532f5..4c77f10c11de4 100644
--- a/app/code/Magento/Customer/Model/Address/CompositeValidator.php
+++ b/app/code/Magento/Customer/Model/Address/CompositeValidator.php
@@ -30,11 +30,11 @@ public function __construct(
      */
     public function validate(AbstractAddress $address)
     {
-        $errors = [];
+        $errors = [[]];
         foreach ($this->validators as $validator) {
-            $errors = array_merge($errors, $validator->validate($address));
+            $errors[] = $validator->validate($address);
         }
 
-        return $errors;
+        return array_merge(...$errors);
     }
 }
diff --git a/app/code/Magento/Customer/Model/Metadata/Form.php b/app/code/Magento/Customer/Model/Metadata/Form.php
index 6df969f625376..85637ebf508b8 100644
--- a/app/code/Magento/Customer/Model/Metadata/Form.php
+++ b/app/code/Magento/Customer/Model/Metadata/Form.php
@@ -9,6 +9,8 @@
 use Magento\Customer\Api\CustomerMetadataInterface;
 
 /**
+ * Customer Form metadata model
+ *
  * @api
  * @since 100.0.2
  */
@@ -361,11 +363,11 @@ public function validateData(array $data)
     {
         $validator = $this->_getValidator($data);
         if (!$validator->isValid(false)) {
-            $messages = [];
+            $messages = [[]];
             foreach ($validator->getMessages() as $errorMessages) {
-                $messages = array_merge($messages, (array)$errorMessages);
+                $messages[] = (array)$errorMessages;
             }
-            return $messages;
+            return array_merge(...$messages);
         }
         return true;
     }
diff --git a/app/code/Magento/Customer/Model/ResourceModel/Address/Attribute/Source/CountryWithWebsites.php b/app/code/Magento/Customer/Model/ResourceModel/Address/Attribute/Source/CountryWithWebsites.php
index ce6278b446302..020067570efb4 100644
--- a/app/code/Magento/Customer/Model/ResourceModel/Address/Attribute/Source/CountryWithWebsites.php
+++ b/app/code/Magento/Customer/Model/ResourceModel/Address/Attribute/Source/CountryWithWebsites.php
@@ -12,18 +12,28 @@
 namespace Magento\Customer\Model\ResourceModel\Address\Attribute\Source;
 
 use Magento\Customer\Model\Config\Share;
+use Magento\Customer\Model\Config\Share as CustomerShareConfig;
 use Magento\Directory\Model\AllowedCountries;
+use Magento\Directory\Model\ResourceModel\Country\Collection as CountryCollection;
+use Magento\Directory\Model\ResourceModel\Country\CollectionFactory as CountryCollectionFactory;
+use Magento\Eav\Model\Entity\Attribute\Source\Table;
+use Magento\Eav\Model\ResourceModel\Entity\Attribute\Option\CollectionFactory as OptionCollectionFactory;
+use Magento\Eav\Model\ResourceModel\Entity\Attribute\OptionFactory as AttrubuteOptionFactory;
 use Magento\Store\Model\ScopeInterface;
+use Magento\Store\Model\StoreManagerInterface;
 
-class CountryWithWebsites extends \Magento\Eav\Model\Entity\Attribute\Source\Table
+/**
+ * Return allowed countries for specified website
+ */
+class CountryWithWebsites extends Table
 {
     /**
-     * @var \Magento\Directory\Model\ResourceModel\Country\CollectionFactory
+     * @var CountryCollectionFactory
      */
     private $countriesFactory;
 
     /**
-     * @var \Magento\Directory\Model\AllowedCountries
+     * @var AllowedCountries
      */
     private $allowedCountriesReader;
 
@@ -33,7 +43,7 @@ class CountryWithWebsites extends \Magento\Eav\Model\Entity\Attribute\Source\Tab
     private $options;
 
     /**
-     * @var \Magento\Store\Model\StoreManagerInterface
+     * @var StoreManagerInterface
      */
     private $storeManager;
 
@@ -43,21 +53,20 @@ class CountryWithWebsites extends \Magento\Eav\Model\Entity\Attribute\Source\Tab
     private $shareConfig;
 
     /**
-     * CountryWithWebsites constructor.
-     * @param \Magento\Eav\Model\ResourceModel\Entity\Attribute\Option\CollectionFactory $attrOptionCollectionFactory
-     * @param \Magento\Eav\Model\ResourceModel\Entity\Attribute\OptionFactory $attrOptionFactory
-     * @param \Magento\Directory\Model\ResourceModel\Country\CollectionFactory $countriesFactory
+     * @param OptionCollectionFactory $attrOptionCollectionFactory
+     * @param AttrubuteOptionFactory $attrOptionFactory
+     * @param CountryCollectionFactory $countriesFactory
      * @param AllowedCountries $allowedCountriesReader
-     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
+     * @param StoreManagerInterface $storeManager
      * @param Share $shareConfig
      */
     public function __construct(
-        \Magento\Eav\Model\ResourceModel\Entity\Attribute\Option\CollectionFactory $attrOptionCollectionFactory,
-        \Magento\Eav\Model\ResourceModel\Entity\Attribute\OptionFactory $attrOptionFactory,
-        \Magento\Directory\Model\ResourceModel\Country\CollectionFactory $countriesFactory,
-        \Magento\Directory\Model\AllowedCountries $allowedCountriesReader,
-        \Magento\Store\Model\StoreManagerInterface $storeManager,
-        \Magento\Customer\Model\Config\Share $shareConfig
+        OptionCollectionFactory $attrOptionCollectionFactory,
+        AttrubuteOptionFactory $attrOptionFactory,
+        CountryCollectionFactory $countriesFactory,
+        AllowedCountries $allowedCountriesReader,
+        StoreManagerInterface $storeManager,
+        CustomerShareConfig $shareConfig
     ) {
         $this->countriesFactory = $countriesFactory;
         $this->allowedCountriesReader = $allowedCountriesReader;
@@ -72,19 +81,22 @@ public function __construct(
     public function getAllOptions($withEmpty = true, $defaultValues = false)
     {
         if (!$this->options) {
-            $allowedCountries = [];
             $websiteIds = [];
 
             if (!$this->shareConfig->isGlobalScope()) {
+                $allowedCountries = [[]];
+
                 foreach ($this->storeManager->getWebsites() as $website) {
                     $countries = $this->allowedCountriesReader
                         ->getAllowedCountries(ScopeInterface::SCOPE_WEBSITE, $website->getId());
-                    $allowedCountries = array_merge($allowedCountries, $countries);
+                    $allowedCountries[] = $countries;
 
                     foreach ($countries as $countryCode) {
                         $websiteIds[$countryCode][] = $website->getId();
                     }
                 }
+
+                $allowedCountries = array_unique(array_merge(...$allowedCountries));
             } else {
                 $allowedCountries = $this->allowedCountriesReader->getAllowedCountries();
             }
@@ -106,7 +118,7 @@ public function getAllOptions($withEmpty = true, $defaultValues = false)
     /**
      * Create Countries Collection with all countries
      *
-     * @return \Magento\Directory\Model\ResourceModel\Country\Collection
+     * @return CountryCollection
      */
     private function createCountriesCollection()
     {
diff --git a/app/code/Magento/CustomerImportExport/Model/Import/Address.php b/app/code/Magento/CustomerImportExport/Model/Import/Address.php
index 1a8859d5bd7bf..55c58282bcae4 100644
--- a/app/code/Magento/CustomerImportExport/Model/Import/Address.php
+++ b/app/code/Magento/CustomerImportExport/Model/Import/Address.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\CustomerImportExport\Model\Import;
 
 use Magento\Customer\Model\ResourceModel\Address\Attribute\Source\CountryWithWebsites as CountryWithWebsitesSource;
@@ -224,7 +225,11 @@ class Address extends AbstractCustomer
      * @array
      */
     protected $validColumnNames = [
-        "region_id", "vat_is_valid", "vat_request_date", "vat_request_id", "vat_request_success"
+        "region_id",
+        "vat_is_valid",
+        "vat_request_date",
+        "vat_request_id",
+        "vat_request_success"
     ];
 
     /**
@@ -507,11 +512,12 @@ public function validateData()
     protected function _importData()
     {
         //Preparing data for mass validation/import.
-        $rows = [];
+        $rows = [[]];
         while ($bunch = $this->_dataSourceModel->getNextBunch()) {
-            $rows = array_merge($rows, $bunch);
+            $rows[] = $bunch;
         }
-        $this->prepareCustomerData($rows);
+
+        $this->prepareCustomerData(array_merge(...$rows));
         unset($bunch, $rows);
         $this->_dataSourceModel->getIterator()->rewind();
 
@@ -586,7 +592,7 @@ protected function _mergeEntityAttributes(array $newAttributes, array $attribute
      * @SuppressWarnings(PHPMD.CyclomaticComplexity)
      * @SuppressWarnings(PHPMD.NPathComplexity)
      */
-    protected function _prepareDataForUpdate(array $rowData):array
+    protected function _prepareDataForUpdate(array $rowData): array
     {
         $email = strtolower($rowData[self::COLUMN_EMAIL]);
         $customerId = $this->_getCustomerId($email, $rowData[self::COLUMN_WEBSITE]);
@@ -636,7 +642,7 @@ protected function _prepareDataForUpdate(array $rowData):array
                 if ($attributeParams['is_static']) {
                     $entityRow[$attributeAlias] = $value;
                 } else {
-                    $attributes[$attributeParams['table']][$addressId][$attributeParams['id']]= $value;
+                    $attributes[$attributeParams['table']][$addressId][$attributeParams['id']] = $value;
                 }
             }
         }
diff --git a/app/code/Magento/CustomerImportExport/Model/Import/Customer.php b/app/code/Magento/CustomerImportExport/Model/Import/Customer.php
index 5e5f309915d99..cf3fa6b0b521f 100644
--- a/app/code/Magento/CustomerImportExport/Model/Import/Customer.php
+++ b/app/code/Magento/CustomerImportExport/Model/Import/Customer.php
@@ -502,8 +502,8 @@ protected function _importData()
     {
         while ($bunch = $this->_dataSourceModel->getNextBunch()) {
             $this->prepareCustomerData($bunch);
-            $entitiesToCreate = [];
-            $entitiesToUpdate = [];
+            $entitiesToCreate = [[]];
+            $entitiesToUpdate = [[]];
             $entitiesToDelete = [];
             $attributesToSave = [];
 
@@ -523,10 +523,10 @@ protected function _importData()
                     );
                 } elseif ($this->getBehavior($rowData) == \Magento\ImportExport\Model\Import::BEHAVIOR_ADD_UPDATE) {
                     $processedData = $this->_prepareDataForUpdate($rowData);
-                    // phpcs:disable Magento2.Performance.ForeachArrayMerge
-                    $entitiesToCreate = array_merge($entitiesToCreate, $processedData[self::ENTITIES_TO_CREATE_KEY]);
-                    $entitiesToUpdate = array_merge($entitiesToUpdate, $processedData[self::ENTITIES_TO_UPDATE_KEY]);
-                    // phpcs:enable
+
+                    $entitiesToCreate[] = $processedData[self::ENTITIES_TO_CREATE_KEY];
+                    $entitiesToUpdate[] = $processedData[self::ENTITIES_TO_UPDATE_KEY];
+
                     foreach ($processedData[self::ATTRIBUTES_TO_SAVE_KEY] as $tableName => $customerAttributes) {
                         if (!isset($attributesToSave[$tableName])) {
                             $attributesToSave[$tableName] = [];
@@ -536,6 +536,10 @@ protected function _importData()
                     }
                 }
             }
+
+            $entitiesToCreate = array_merge(...$entitiesToCreate);
+            $entitiesToUpdate = array_merge(...$entitiesToUpdate);
+
             $this->updateItemsCounterStats($entitiesToCreate, $entitiesToUpdate, $entitiesToDelete);
             /**
              * Save prepared data
diff --git a/app/code/Magento/Deploy/Package/Package.php b/app/code/Magento/Deploy/Package/Package.php
index 423f3072c4620..4821d32454675 100644
--- a/app/code/Magento/Deploy/Package/Package.php
+++ b/app/code/Magento/Deploy/Package/Package.php
@@ -443,12 +443,11 @@ public function getResultMap()
      */
     public function getParentMap()
     {
-        $map = [];
+        $map = [[]];
         foreach ($this->getParentPackages() as $parentPackage) {
-            // phpcs:ignore Magento2.Performance.ForeachArrayMerge.ForeachArrayMerge
-            $map = array_merge($map, $parentPackage->getMap());
+            $map[] = $parentPackage->getMap();
         }
-        return $map;
+        return array_merge(...$map);
     }
 
     /**
@@ -462,10 +461,8 @@ public function getParentFiles($type = null)
         $files = [[]];
         foreach ($this->getParentPackages() as $parentPackage) {
             if ($type === null) {
-                // phpcs:ignore Magento2.Performance.ForeachArrayMerge.ForeachArrayMerge
                 $files[] = $parentPackage->getFiles();
             } else {
-                // phpcs:ignore Magento2.Performance.ForeachArrayMerge.ForeachArrayMerge
                 $files[] = $parentPackage->getFilesByType($type);
             }
         }
diff --git a/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php b/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
index b3461e3ab4a22..42775a2e2f6bf 100644
--- a/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
+++ b/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
@@ -3,12 +3,14 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Deploy\Package\Processor\PreProcessor;
 
 use Magento\Deploy\Console\DeployStaticOptions;
 use Magento\Deploy\Package\Package;
 use Magento\Deploy\Package\PackageFile;
 use Magento\Deploy\Package\Processor\ProcessorInterface;
+use Magento\Framework\Css\PreProcessor\Instruction\Import;
 use Magento\Framework\Filesystem;
 use Magento\Framework\Filesystem\Directory\ReadInterface;
 use Magento\Framework\App\Filesystem\DirectoryList;
@@ -92,8 +94,7 @@ public function process(Package $package, array $options)
     }
 
     /**
-     * Checks if there are imports of CSS files or images within the given CSS file
-     * which exists in the current package
+     * Checks if there are imports of CSS files or images within the given CSS file which exists in the current package
      *
      * @param PackageFile $parentFile
      * @param Package $package
@@ -126,6 +127,7 @@ private function hasOverrides(PackageFile $parentFile, Package $package)
      * @param string $filePath
      * @param string $fullPath
      * @return void
+     * phpcs:disable Magento2.Functions.DiscouragedFunction
      */
     private function buildMap($packagePath, $filePath, $fullPath)
     {
@@ -141,11 +143,7 @@ private function buildMap($packagePath, $filePath, $fullPath)
                     $packagePath . '/' . pathinfo($filePath, PATHINFO_DIRNAME) . '/' . $matchContent['path']
                 );
             };
-            preg_replace_callback(
-                \Magento\Framework\Css\PreProcessor\Instruction\Import::REPLACE_PATTERN,
-                $callback,
-                $content
-            );
+            preg_replace_callback(Import::REPLACE_PATTERN, $callback, $content);
 
             preg_match_all(CssResolver::REGEX_CSS_RELATIVE_URLS, $content, $matches);
             if (!empty($matches[0]) && !empty($matches[1])) {
@@ -177,6 +175,7 @@ private function buildMap($packagePath, $filePath, $fullPath)
      *
      * @param string $fileName
      * @return array
+     * phpcs:disable Magento2.Performance.ForeachArrayMerge
      */
     private function collectFileMap($fileName)
     {
diff --git a/app/code/Magento/Developer/Console/Command/XmlCatalogGenerateCommand.php b/app/code/Magento/Developer/Console/Command/XmlCatalogGenerateCommand.php
index 323b8e1016291..9e473ccaa2d92 100644
--- a/app/code/Magento/Developer/Console/Command/XmlCatalogGenerateCommand.php
+++ b/app/code/Magento/Developer/Console/Command/XmlCatalogGenerateCommand.php
@@ -6,7 +6,12 @@
 
 namespace Magento\Developer\Console\Command;
 
+use Magento\Developer\Model\XmlCatalog\Format\FormatInterface;
+use Magento\Framework\App\Utility\Files;
+use Magento\Framework\Config\Dom\UrnResolver;
+use Magento\Framework\Console\Cli;
 use Magento\Framework\Exception\InputException;
+use Magento\Framework\Filesystem\Directory\ReadFactory;
 use Symfony\Component\Console\Command\Command;
 use Symfony\Component\Console\Input\InputArgument;
 use Symfony\Component\Console\Input\InputOption;
@@ -34,37 +39,37 @@ class XmlCatalogGenerateCommand extends Command
     const IDE_FILE_PATH_ARGUMENT = 'path';
 
     /**
-     * @var \Magento\Framework\App\Utility\Files
+     * @var Files
      */
     private $filesUtility;
 
     /**
-     * @var \Magento\Framework\Config\Dom\UrnResolver
+     * @var UrnResolver
      */
     private $urnResolver;
 
     /**
-     * @var \Magento\Framework\Filesystem\Directory\ReadFactory
+     * @var ReadFactory
      */
     private $readFactory;
 
     /**
      * Supported formats
      *
-     * @var \Magento\Developer\Model\XmlCatalog\Format\FormatInterface[]
+     * @var FormatInterface[]
      */
     private $formats;
 
     /**
-     * @param \Magento\Framework\App\Utility\Files $filesUtility
-     * @param \Magento\Framework\Config\Dom\UrnResolver $urnResolver
-     * @param \Magento\Framework\Filesystem\Directory\ReadFactory $readFactory
-     * @param \Magento\Developer\Model\XmlCatalog\Format\FormatInterface[] $formats
+     * @param Files $filesUtility
+     * @param UrnResolver $urnResolver
+     * @param ReadFactory $readFactory
+     * @param FormatInterface[] $formats
      */
     public function __construct(
-        \Magento\Framework\App\Utility\Files $filesUtility,
-        \Magento\Framework\Config\Dom\UrnResolver $urnResolver,
-        \Magento\Framework\Filesystem\Directory\ReadFactory $readFactory,
+        Files $filesUtility,
+        UrnResolver $urnResolver,
+        ReadFactory $readFactory,
         array $formats = []
     ) {
         $this->filesUtility = $filesUtility;
@@ -75,7 +80,7 @@ public function __construct(
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      */
     protected function configure()
     {
@@ -86,7 +91,7 @@ protected function configure()
                     self::IDE_OPTION,
                     null,
                     InputOption::VALUE_REQUIRED,
-                    'Format in which catalog will be generated. Supported: ['.
+                    'Format in which catalog will be generated. Supported: [' .
                     implode(', ', $this->getSupportedFormats()) . ']',
                     'phpstorm'
                 ),
@@ -111,19 +116,21 @@ private function getUrnDictionary(OutputInterface $output)
         $files = $this->filesUtility->getXmlCatalogFiles('*.xml');
         $files = array_merge($files, $this->filesUtility->getXmlCatalogFiles('*.xsd'));
 
-        $urns = [];
+        $urns = [[]];
         foreach ($files as $file) {
+            // phpcs:ignore Magento2.Functions.DiscouragedFunction
             $fileDir = dirname($file[0]);
+            // phpcs:ignore Magento2.Functions.DiscouragedFunction
             $fileName = basename($file[0]);
             $read = $this->readFactory->create($fileDir);
             $content = $read->readFile($fileName);
             $matches = [];
             preg_match_all('/schemaLocation="(urn\:magento\:[^"]*)"/i', $content, $matches);
             if (isset($matches[1])) {
-                $urns = array_merge($urns, $matches[1]);
+                $urns[] = $matches[1];
             }
         }
-        $urns = array_unique($urns);
+        $urns = array_unique(array_merge(...$urns));
         $paths = [];
         foreach ($urns as $urn) {
             try {
@@ -139,27 +146,32 @@ private function getUrnDictionary(OutputInterface $output)
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
+     *
      * @throws \InvalidArgumentException
      */
     protected function execute(InputInterface $input, OutputInterface $output)
     {
         $ideName = $input->getOption(self::IDE_OPTION);
-        $ideFilePath = $input->getArgument(self::IDE_FILE_PATH_ARGUMENT);
 
+        $ideFilePath = $input->getArgument(self::IDE_FILE_PATH_ARGUMENT);
         $urnDictionary = $this->getUrnDictionary($output);
-        if ($formatter = $this->getFormatters($ideName)) {
-            $formatter->generateCatalog($urnDictionary, $ideFilePath);
-        } else {
+
+        $formatter = $this->getFormatters($ideName);
+        if (!$formatter instanceof FormatInterface) {
             throw new InputException(__("Format for IDE '%1' is not supported", $ideName));
         }
+
+        $formatter->generateCatalog($urnDictionary, $ideFilePath);
+
+        return Cli::RETURN_SUCCESS;
     }
 
     /**
      * Get formatter based on format
      *
      * @param string $format
-     * @return \Magento\Developer\Model\XmlCatalog\Format\FormatInterface|false
+     * @return FormatInterface|false
      */
     private function getFormatters($format)
     {
diff --git a/app/code/Magento/Directory/Model/AllowedCountries.php b/app/code/Magento/Directory/Model/AllowedCountries.php
index b9f2d829dd1a6..2ceeb70ba5b01 100644
--- a/app/code/Magento/Directory/Model/AllowedCountries.php
+++ b/app/code/Magento/Directory/Model/AllowedCountries.php
@@ -62,13 +62,11 @@ public function getAllowedCountries(
         switch ($scope) {
             case ScopeInterface::SCOPE_WEBSITES:
             case ScopeInterface::SCOPE_STORES:
-                $allowedCountries = [];
+                $allowedCountries = [[]];
                 foreach ($scopeCode as $singleFilter) {
-                    $allowedCountries = array_merge(
-                        $allowedCountries,
-                        $this->getCountriesFromConfig($this->getSingleScope($scope), $singleFilter)
-                    );
+                    $allowedCountries[] = $this->getCountriesFromConfig($this->getSingleScope($scope), $singleFilter);
                 }
+                $allowedCountries = array_merge(...$allowedCountries);
                 break;
             default:
                 $allowedCountries = $this->getCountriesFromConfig($scope, $scopeCode);
diff --git a/app/code/Magento/Eav/Block/Adminhtml/Attribute/Edit/Options/Options.php b/app/code/Magento/Eav/Block/Adminhtml/Attribute/Edit/Options/Options.php
index 7af7bf447c45a..69f417e1ea732 100644
--- a/app/code/Magento/Eav/Block/Adminhtml/Attribute/Edit/Options/Options.php
+++ b/app/code/Magento/Eav/Block/Adminhtml/Attribute/Edit/Options/Options.php
@@ -152,10 +152,10 @@ protected function _prepareOptionValues(
             $inputType = '';
         }
 
-        $values = [];
+        $values = [[]];
         $isSystemAttribute = is_array($optionCollection);
         if ($isSystemAttribute) {
-            $values = $this->getPreparedValues($optionCollection, $isSystemAttribute, $inputType, $defaultValues);
+            $values[] = $this->getPreparedValues($optionCollection, $isSystemAttribute, $inputType, $defaultValues);
         } else {
             $optionCollection->setPageSize(200);
             $pageCount = $optionCollection->getLastPageNumber();
@@ -163,15 +163,12 @@ protected function _prepareOptionValues(
             while ($currentPage <= $pageCount) {
                 $optionCollection->clear();
                 $optionCollection->setCurPage($currentPage);
-                $values = array_merge(
-                    $values,
-                    $this->getPreparedValues($optionCollection, $isSystemAttribute, $inputType, $defaultValues)
-                );
+                $values[] = $this->getPreparedValues($optionCollection, $isSystemAttribute, $inputType, $defaultValues);
                 $currentPage++;
             }
         }
 
-        return $values;
+        return array_merge(...$values);
     }
 
     /**
diff --git a/app/code/Magento/Eav/Model/Form.php b/app/code/Magento/Eav/Model/Form.php
index a34b53eede354..074c6cf46a2f4 100644
--- a/app/code/Magento/Eav/Model/Form.php
+++ b/app/code/Magento/Eav/Model/Form.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Eav\Model;
 
 use Magento\Framework\App\RequestInterface;
@@ -10,6 +11,7 @@
 /**
  * EAV Entity Form Model
  *
+ * phpcs:disable Magento2.Classes.AbstractApi
  * @api
  * @SuppressWarnings(PHPMD.TooManyFields)
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
@@ -274,8 +276,8 @@ public function getStore()
     /**
      * Return current form code
      *
-     * @throws \Magento\Framework\Exception\LocalizedException
      * @return string
+     * @throws \Magento\Framework\Exception\LocalizedException
      */
     public function getFormCode()
     {
@@ -303,8 +305,8 @@ public function getEntityType()
     /**
      * Return current entity instance
      *
-     * @throws \Magento\Framework\Exception\LocalizedException
      * @return \Magento\Framework\Model\AbstractModel
+     * @throws \Magento\Framework\Exception\LocalizedException
      */
     public function getEntity()
     {
@@ -485,11 +487,11 @@ public function validateData(array $data)
     {
         $validator = $this->_getValidator($data);
         if (!$validator->isValid($this->getEntity())) {
-            $messages = [];
+            $messages = [[]];
             foreach ($validator->getMessages() as $errorMessages) {
-                $messages = array_merge($messages, (array)$errorMessages);
+                $messages[] = (array)$errorMessages;
             }
-            return $messages;
+            return array_merge(...$messages);
         }
         return true;
     }
diff --git a/app/code/Magento/Elasticsearch/Model/Adapter/FieldMapper/Product/CompositeFieldProvider.php b/app/code/Magento/Elasticsearch/Model/Adapter/FieldMapper/Product/CompositeFieldProvider.php
index 8038c8c05bc1c..b276b67ff7fba 100644
--- a/app/code/Magento/Elasticsearch/Model/Adapter/FieldMapper/Product/CompositeFieldProvider.php
+++ b/app/code/Magento/Elasticsearch/Model/Adapter/FieldMapper/Product/CompositeFieldProvider.php
@@ -40,12 +40,12 @@ public function __construct(array $providers)
      */
     public function getFields(array $context = []): array
     {
-        $allAttributes = [];
+        $allAttributes = [[]];
 
         foreach ($this->providers as $provider) {
-            $allAttributes = array_merge($allAttributes, $provider->getFields($context));
+            $allAttributes[] = $provider->getFields($context);
         }
 
-        return $allAttributes;
+        return array_merge(...$allAttributes);
     }
 }
diff --git a/app/code/Magento/GroupedProduct/Block/Cart/Item/Renderer/Grouped.php b/app/code/Magento/GroupedProduct/Block/Cart/Item/Renderer/Grouped.php
index e85d4eeca730a..197be38fb7f5f 100644
--- a/app/code/Magento/GroupedProduct/Block/Cart/Item/Renderer/Grouped.php
+++ b/app/code/Magento/GroupedProduct/Block/Cart/Item/Renderer/Grouped.php
@@ -45,10 +45,10 @@ public function getGroupedProduct()
      */
     public function getIdentities()
     {
-        $identities = parent::getIdentities();
+        $identities = [parent::getIdentities()];
         if ($this->getItem()) {
-            $identities = array_merge($identities, $this->getGroupedProduct()->getIdentities());
+            $identities[] = $this->getGroupedProduct()->getIdentities();
         }
-        return $identities;
+        return array_merge(...$identities);
     }
 }
diff --git a/app/code/Magento/GroupedProduct/Block/Stockqty/Type/Grouped.php b/app/code/Magento/GroupedProduct/Block/Stockqty/Type/Grouped.php
index f666cf8d90af9..97dc90ec93493 100644
--- a/app/code/Magento/GroupedProduct/Block/Stockqty/Type/Grouped.php
+++ b/app/code/Magento/GroupedProduct/Block/Stockqty/Type/Grouped.php
@@ -32,10 +32,10 @@ protected function _getChildProducts()
      */
     public function getIdentities()
     {
-        $identities = [];
+        $identities = [[]];
         foreach ($this->getChildProducts() as $item) {
-            $identities = array_merge($identities, $item->getIdentities());
+            $identities[] = $item->getIdentities();
         }
-        return $identities;
+        return array_merge(...$identities);
     }
 }
diff --git a/app/code/Magento/ImportExport/Model/Import/ErrorProcessing/ProcessingErrorAggregator.php b/app/code/Magento/ImportExport/Model/Import/ErrorProcessing/ProcessingErrorAggregator.php
index 028bf2c464d4b..5ea6227231543 100644
--- a/app/code/Magento/ImportExport/Model/Import/ErrorProcessing/ProcessingErrorAggregator.php
+++ b/app/code/Magento/ImportExport/Model/Import/ErrorProcessing/ProcessingErrorAggregator.php
@@ -237,16 +237,12 @@ public function hasFatalExceptions()
      */
     public function getAllErrors()
     {
-        $result = [];
-        if (empty($this->items)) {
-            return $result;
-        }
-
-        foreach (array_values($this->items['rows']) as $errors) {
-            $result = array_merge($result, $errors);
+        if (empty($this->items) || empty($this->items['rows'])) {
+            return [];
         }
 
-        return $result;
+        $errors = array_values($this->items['rows']);
+        return array_merge(...$errors);
     }
 
     /**
@@ -257,14 +253,14 @@ public function getAllErrors()
      */
     public function getErrorsByCode(array $codes)
     {
-        $result = [];
+        $result = [[]];
         foreach ($codes as $code) {
             if (isset($this->items['codes'][$code])) {
-                $result = array_merge($result, $this->items['codes'][$code]);
+                $result[] = $this->items['codes'][$code];
             }
         }
 
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
diff --git a/app/code/Magento/Integration/Block/Adminhtml/Integration/Activate/Permissions/Tab/Webapi.php b/app/code/Magento/Integration/Block/Adminhtml/Integration/Activate/Permissions/Tab/Webapi.php
index 3c719a5814615..2d323fea34e7d 100644
--- a/app/code/Magento/Integration/Block/Adminhtml/Integration/Activate/Permissions/Tab/Webapi.php
+++ b/app/code/Magento/Integration/Block/Adminhtml/Integration/Activate/Permissions/Tab/Webapi.php
@@ -105,7 +105,7 @@ protected function _construct()
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      */
     public function canShowTab()
     {
@@ -116,7 +116,7 @@ public function canShowTab()
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      *
      * @codeCoverageIgnore
      */
@@ -126,7 +126,7 @@ public function getTabLabel()
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      *
      * @codeCoverageIgnore
      */
@@ -136,7 +136,7 @@ public function getTabTitle()
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      *
      * @codeCoverageIgnore
      */
@@ -222,13 +222,13 @@ public function isTreeEmpty()
      */
     protected function _getAllResourceIds(array $resources)
     {
-        $resourceIds = [];
+        $resourceIds = [[]];
         foreach ($resources as $resource) {
-            $resourceIds[] = $resource['id'];
+            $resourceIds[] = [$resource['id']];
             if (isset($resource['children'])) {
-                $resourceIds = array_merge($resourceIds, $this->_getAllResourceIds($resource['children']));
+                $resourceIds[] = $this->_getAllResourceIds($resource['children']);
             }
         }
-        return $resourceIds;
+        return array_merge(...$resourceIds);
     }
 }
diff --git a/app/code/Magento/Multishipping/Test/Unit/Plugin/DisableMultishippingModeTest.php b/app/code/Magento/Multishipping/Test/Unit/Plugin/DisableMultishippingModeTest.php
index 02ae1a70ce801..829d5a9ce2763 100644
--- a/app/code/Magento/Multishipping/Test/Unit/Plugin/DisableMultishippingModeTest.php
+++ b/app/code/Magento/Multishipping/Test/Unit/Plugin/DisableMultishippingModeTest.php
@@ -16,7 +16,7 @@
 use Magento\Quote\Model\Quote;
 
 /**
- * Class DisableMultishippingModeTest
+ * Set of Unit Tets to cover DisableMultishippingMode
  */
 class DisableMultishippingModeTest extends \PHPUnit\Framework\TestCase
 {
@@ -59,10 +59,10 @@ protected function setUp()
     public function testExecuteTurnsOffMultishippingModeOnMultishippingQuote(): void
     {
         $subject = $this->createMock(Index::class);
-        $extensionAttributes = $this->createPartialMock(
-            CartExtensionInterface::class,
-            ['setShippingAssignments', 'getShippingAssignments']
-        );
+        $extensionAttributes = $this->getMockBuilder(CartExtensionInterface::class)
+            ->disableOriginalConstructor()
+            ->setMethods(['setShippingAssignments', 'getShippingAssignments'])
+            ->getMockForAbstractClass();
         $extensionAttributes->method('getShippingAssignments')
             ->willReturn(
                 $this->createMock(ShippingAssignmentInterface::class)
diff --git a/app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php b/app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php
index 88619673ad425..b4d9cb2d5e076 100644
--- a/app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php
+++ b/app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php
@@ -3,8 +3,15 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\PageCache\Model\Layout;
 
+use Magento\Framework\App\MaintenanceMode;
+use Magento\Framework\App\ResponseInterface;
+use Magento\Framework\DataObject\IdentityInterface;
+use Magento\Framework\View\Layout;
+use Magento\PageCache\Model\Config;
+
 /**
  * Class LayoutPlugin
  *
@@ -13,31 +20,31 @@
 class LayoutPlugin
 {
     /**
-     * @var \Magento\PageCache\Model\Config
+     * @var Config
      */
     protected $config;
 
     /**
-     * @var \Magento\Framework\App\ResponseInterface
+     * @var ResponseInterface
      */
     protected $response;
 
     /**
-     * @var \Magento\Framework\App\MaintenanceMode
+     * @var MaintenanceMode
      */
     private $maintenanceMode;
 
     /**
      * Constructor
      *
-     * @param \Magento\Framework\App\ResponseInterface $response
-     * @param \Magento\PageCache\Model\Config          $config
-     * @param \Magento\Framework\App\MaintenanceMode   $maintenanceMode
+     * @param ResponseInterface $response
+     * @param Config $config
+     * @param MaintenanceMode $maintenanceMode
      */
     public function __construct(
-        \Magento\Framework\App\ResponseInterface $response,
-        \Magento\PageCache\Model\Config $config,
-        \Magento\Framework\App\MaintenanceMode $maintenanceMode
+        ResponseInterface $response,
+        Config $config,
+        MaintenanceMode $maintenanceMode
     ) {
         $this->response = $response;
         $this->config = $config;
@@ -49,11 +56,11 @@ public function __construct(
      *
      * We have to set public headers in order to tell Varnish and Builtin app that page should be cached
      *
-     * @param \Magento\Framework\View\Layout $subject
+     * @param Layout $subject
      * @param mixed $result
      * @return mixed
      */
-    public function afterGenerateXml(\Magento\Framework\View\Layout $subject, $result)
+    public function afterGenerateXml(Layout $subject, $result)
     {
         if ($subject->isCacheable() && !$this->maintenanceMode->isOn() && $this->config->isEnabled()) {
             $this->response->setPublicHeaders($this->config->getTtl());
@@ -64,26 +71,25 @@ public function afterGenerateXml(\Magento\Framework\View\Layout $subject, $resul
     /**
      * Retrieve all identities from blocks for further cache invalidation
      *
-     * @param \Magento\Framework\View\Layout $subject
+     * @param Layout $subject
      * @param mixed $result
      * @return mixed
      */
-    public function afterGetOutput(\Magento\Framework\View\Layout $subject, $result)
+    public function afterGetOutput(Layout $subject, $result)
     {
         if ($subject->isCacheable() && $this->config->isEnabled()) {
-            $tags = [];
+            $tags = [[]];
             foreach ($subject->getAllBlocks() as $block) {
-                if ($block instanceof \Magento\Framework\DataObject\IdentityInterface) {
+                if ($block instanceof IdentityInterface) {
                     $isEsiBlock = $block->getTtl() > 0;
-                    $isVarnish = $this->config->getType() == \Magento\PageCache\Model\Config::VARNISH;
+                    $isVarnish = $this->config->getType() == Config::VARNISH;
                     if ($isVarnish && $isEsiBlock) {
                         continue;
                     }
-                    // phpcs:ignore
-                    $tags = array_merge($tags, $block->getIdentities());
+                    $tags[] = $block->getIdentities();
                 }
             }
-            $tags = array_unique($tags);
+            $tags = array_unique(array_merge(...$tags));
             $this->response->setHeader('X-Magento-Tags', implode(',', $tags));
         }
         return $result;
diff --git a/app/code/Magento/Payment/Gateway/Validator/ValidatorComposite.php b/app/code/Magento/Payment/Gateway/Validator/ValidatorComposite.php
index 8ea97d31ed4d9..f96c08a9605a8 100644
--- a/app/code/Magento/Payment/Gateway/Validator/ValidatorComposite.php
+++ b/app/code/Magento/Payment/Gateway/Validator/ValidatorComposite.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Payment\Gateway\Validator;
 
 use Magento\Framework\ObjectManager\TMap;
@@ -57,26 +58,25 @@ public function __construct(
     public function validate(array $validationSubject)
     {
         $isValid = true;
-        $failsDescriptionAggregate = [];
-        $errorCodesAggregate = [];
+        $failsDescriptionAggregate = [[]];
+        $errorCodesAggregate = [[]];
         foreach ($this->validators as $key => $validator) {
             $result = $validator->validate($validationSubject);
             if (!$result->isValid()) {
                 $isValid = false;
-                $failsDescriptionAggregate = array_merge(
-                    $failsDescriptionAggregate,
-                    $result->getFailsDescription()
-                );
-                $errorCodesAggregate = array_merge(
-                    $errorCodesAggregate,
-                    $result->getErrorCodes()
-                );
+                $failsDescriptionAggregate[] = $result->getFailsDescription();
+                $errorCodesAggregate[] = $result->getErrorCodes();
+
                 if (!empty($this->chainBreakingValidators[$key])) {
                     break;
                 }
             }
         }
 
-        return $this->createResult($isValid, $failsDescriptionAggregate, $errorCodesAggregate);
+        return $this->createResult(
+            $isValid,
+            array_merge(...$failsDescriptionAggregate),
+            array_merge(...$errorCodesAggregate)
+        );
     }
 }
diff --git a/app/code/Magento/Paypal/Model/Config/Structure/PaymentSectionModifier.php b/app/code/Magento/Paypal/Model/Config/Structure/PaymentSectionModifier.php
index f1c414e6faa9b..61410499e956e 100644
--- a/app/code/Magento/Paypal/Model/Config/Structure/PaymentSectionModifier.php
+++ b/app/code/Magento/Paypal/Model/Config/Structure/PaymentSectionModifier.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Paypal\Model\Config\Structure;
 
 /**
@@ -60,9 +61,9 @@ public function modify(array $initialStructure)
                         unset($childData['children'][$moveInstruction['section']]);
                         unset($moveInstruction['data']['displayIn']);
                         $changedStructure
-                            [$moveInstruction['parent']]
-                                ['children']
-                                    [$moveInstruction['section']] = $moveInstruction['data'];
+                        [$moveInstruction['parent']]
+                        ['children']
+                        [$moveInstruction['section']] = $moveInstruction['data'];
                     }
                 }
                 if (!isset($moveInstructions[$childSection])) {
@@ -83,7 +84,7 @@ public function modify(array $initialStructure)
      */
     private function getMoveInstructions($section, $data)
     {
-        $moved = [];
+        $moved = [[]];
 
         if (array_key_exists('children', $data)) {
             foreach ($data['children'] as $childSection => $childData) {
@@ -91,23 +92,20 @@ private function getMoveInstructions($section, $data)
                 if (isset($movedChildren[$childSection])) {
                     unset($data['children'][$childSection]);
                 }
-                $moved = array_merge($moved, $movedChildren);
+                $moved[] = $movedChildren;
             }
         }
 
         if (isset($data['displayIn']) && in_array($data['displayIn'], self::$specialGroups)) {
-            $moved = array_merge(
-                [
-                    $section => [
+            $moved[] = [
+                $section => [
                     'parent' => $data['displayIn'],
                     'section' => $section,
                     'data' => $data
-                    ]
-                ],
-                $moved
-            );
+                ]
+            ];
         }
 
-        return $moved;
+        return array_merge(...$moved);
     }
 }
diff --git a/app/code/Magento/QuoteGraphQl/Model/Cart/BuyRequest/BuyRequestBuilder.php b/app/code/Magento/QuoteGraphQl/Model/Cart/BuyRequest/BuyRequestBuilder.php
index 90f32e96a5fde..c14cc1324732c 100644
--- a/app/code/Magento/QuoteGraphQl/Model/Cart/BuyRequest/BuyRequestBuilder.php
+++ b/app/code/Magento/QuoteGraphQl/Model/Cart/BuyRequest/BuyRequestBuilder.php
@@ -45,11 +45,11 @@ public function __construct(
      */
     public function build(array $cartItemData): DataObject
     {
-        $requestData = [];
+        $requestData = [[]];
         foreach ($this->providers as $provider) {
-            $requestData = array_merge($requestData, $provider->execute($cartItemData));
+            $requestData[] = $provider->execute($cartItemData);
         }
 
-        return $this->dataObjectFactory->create(['data' => $requestData]);
+        return $this->dataObjectFactory->create(['data' => array_merge(...$requestData)]);
     }
 }
diff --git a/app/code/Magento/Reports/Block/Product/Viewed.php b/app/code/Magento/Reports/Block/Product/Viewed.php
index 5f9fdf8533523..ba4d03182213a 100644
--- a/app/code/Magento/Reports/Block/Product/Viewed.php
+++ b/app/code/Magento/Reports/Block/Product/Viewed.php
@@ -59,8 +59,7 @@ public function getCount()
     }
 
     /**
-     * Prepare to html
-     * check has viewed products
+     * Prepare to html check has viewed products
      *
      * @return string
      */
@@ -77,10 +76,10 @@ protected function _toHtml()
      */
     public function getIdentities()
     {
-        $identities = [];
+        $identities = [[]];
         foreach ($this->getItemsCollection() as $item) {
-            $identities = array_merge($identities, $item->getIdentities());
+            $identities[] = $item->getIdentities();
         }
-        return $identities;
+        return array_merge(...$identities);
     }
 }
diff --git a/app/code/Magento/Sales/Block/Adminhtml/Items/Column/DefaultColumn.php b/app/code/Magento/Sales/Block/Adminhtml/Items/Column/DefaultColumn.php
index 80dcce2fd1be4..efef617acf900 100644
--- a/app/code/Magento/Sales/Block/Adminhtml/Items/Column/DefaultColumn.php
+++ b/app/code/Magento/Sales/Block/Adminhtml/Items/Column/DefaultColumn.php
@@ -68,19 +68,19 @@ public function getItem()
      */
     public function getOrderOptions()
     {
-        $result = [];
+        $result = [[]];
         if ($options = $this->getItem()->getProductOptions()) {
             if (isset($options['options'])) {
-                $result = array_merge($result, $options['options']);
+                $result[] = $options['options'];
             }
             if (isset($options['additional_options'])) {
-                $result = array_merge($result, $options['additional_options']);
+                $result[] = $options['additional_options'];
             }
             if (!empty($options['attributes_info'])) {
-                $result = array_merge($options['attributes_info'], $result);
+                $result[] = $options['attributes_info'];
             }
         }
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
diff --git a/app/code/Magento/Sales/Block/Order/Email/Items/DefaultItems.php b/app/code/Magento/Sales/Block/Order/Email/Items/DefaultItems.php
index 0aa4f807700b6..064405daf89a8 100644
--- a/app/code/Magento/Sales/Block/Order/Email/Items/DefaultItems.php
+++ b/app/code/Magento/Sales/Block/Order/Email/Items/DefaultItems.php
@@ -29,27 +29,31 @@ public function getOrder()
     }
 
     /**
+     * Returns Items options as array
+     *
      * @return array
      */
     public function getItemOptions()
     {
-        $result = [];
+        $result = [[]];
         if ($options = $this->getItem()->getOrderItem()->getProductOptions()) {
             if (isset($options['options'])) {
-                $result = array_merge($result, $options['options']);
+                $result[] = $options['options'];
             }
             if (isset($options['additional_options'])) {
-                $result = array_merge($result, $options['additional_options']);
+                $result[] = $options['additional_options'];
             }
             if (isset($options['attributes_info'])) {
-                $result = array_merge($result, $options['attributes_info']);
+                $result[] = $options['attributes_info'];
             }
         }
 
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
+     * Formats the value in HTML
+     *
      * @param string|array $value
      * @return string
      */
@@ -70,7 +74,9 @@ public function getValueHtml($value)
     }
 
     /**
-     * @param mixed $item
+     * Returns Product SKU for Item provided
+     *
+     * @param OrderItem $item
      * @return mixed
      */
     public function getSku($item)
diff --git a/app/code/Magento/Sales/Block/Order/Email/Items/Order/DefaultOrder.php b/app/code/Magento/Sales/Block/Order/Email/Items/Order/DefaultOrder.php
index ae2dec6aefa6e..0291a1275c350 100644
--- a/app/code/Magento/Sales/Block/Order/Email/Items/Order/DefaultOrder.php
+++ b/app/code/Magento/Sales/Block/Order/Email/Items/Order/DefaultOrder.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Sales\Block\Order\Email\Items\Order;
 
 use Magento\Sales\Model\Order\Item as OrderItem;
@@ -27,48 +28,49 @@ public function getOrder()
     }
 
     /**
+     * Returns array of Item options
+     *
      * @return array
      */
     public function getItemOptions()
     {
-        $result = [];
+        $result = [[]];
         if ($options = $this->getItem()->getProductOptions()) {
             if (isset($options['options'])) {
-                $result = array_merge($result, $options['options']);
+                $result[] = $options['options'];
             }
             if (isset($options['additional_options'])) {
-                $result = array_merge($result, $options['additional_options']);
+                $result[] = $options['additional_options'];
             }
             if (isset($options['attributes_info'])) {
-                $result = array_merge($result, $options['attributes_info']);
+                $result[] = $options['attributes_info'];
             }
         }
 
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
+     * Formats the value in HTML
+     *
      * @param string|array $value
      * @return string
      */
     public function getValueHtml($value)
     {
         if (is_array($value)) {
-            return sprintf(
-                '%d',
-                $value['qty']
-            ) . ' x ' . $this->escapeHtml(
-                $value['title']
-            ) . " " . $this->getItem()->getOrder()->formatPrice(
-                $value['price']
-            );
+            return sprintf('%d', $value['qty'])
+                . ' x ' . $this->escapeHtml($value['title'])
+                . " " . $this->getItem()->getOrder()->formatPrice($value['price']);
         } else {
             return $this->escapeHtml($value);
         }
     }
 
     /**
-     * @param mixed $item
+     * Returns Product SKU for Item provided
+     *
+     * @param OrderItem $item
      * @return mixed
      */
     public function getSku($item)
diff --git a/app/code/Magento/Sales/Block/Order/Item/Renderer/DefaultRenderer.php b/app/code/Magento/Sales/Block/Order/Item/Renderer/DefaultRenderer.php
index 2e119d0bf887a..bca6d49760d9a 100644
--- a/app/code/Magento/Sales/Block/Order/Item/Renderer/DefaultRenderer.php
+++ b/app/code/Magento/Sales/Block/Order/Item/Renderer/DefaultRenderer.php
@@ -6,7 +6,12 @@
 
 namespace Magento\Sales\Block\Order\Item\Renderer;
 
-use Magento\Sales\Model\Order\CreditMemo\Item as CreditMemoItem;
+use Magento\Catalog\Model\Product\OptionFactory;
+use Magento\Framework\DataObject;
+use Magento\Framework\Stdlib\StringUtils;
+use Magento\Framework\View\Element\AbstractBlock;
+use Magento\Framework\View\Element\Template\Context;
+use Magento\Sales\Model\Order\Creditmemo\Item as CreditMemoItem;
 use Magento\Sales\Model\Order\Invoice\Item as InvoiceItem;
 use Magento\Sales\Model\Order\Item as OrderItem;
 
@@ -21,25 +26,25 @@ class DefaultRenderer extends \Magento\Framework\View\Element\Template
     /**
      * Magento string lib
      *
-     * @var \Magento\Framework\Stdlib\StringUtils
+     * @var StringUtils
      */
     protected $string;
 
     /**
-     * @var \Magento\Catalog\Model\Product\OptionFactory
+     * @var OptionFactory
      */
     protected $_productOptionFactory;
 
     /**
-     * @param \Magento\Framework\View\Element\Template\Context $context
-     * @param \Magento\Framework\Stdlib\StringUtils $string
-     * @param \Magento\Catalog\Model\Product\OptionFactory $productOptionFactory
+     * @param Context $context
+     * @param StringUtils $string
+     * @param OptionFactory $productOptionFactory
      * @param array $data
      */
     public function __construct(
-        \Magento\Framework\View\Element\Template\Context $context,
-        \Magento\Framework\Stdlib\StringUtils $string,
-        \Magento\Catalog\Model\Product\OptionFactory $productOptionFactory,
+        Context $context,
+        StringUtils $string,
+        OptionFactory $productOptionFactory,
         array $data = []
     ) {
         $this->string = $string;
@@ -50,10 +55,10 @@ public function __construct(
     /**
      * Set item.
      *
-     * @param \Magento\Framework\DataObject $item
+     * @param DataObject $item
      * @return $this
      */
-    public function setItem(\Magento\Framework\DataObject $item)
+    public function setItem(DataObject $item)
     {
         $this->setData('item', $item);
         return $this;
@@ -86,7 +91,7 @@ public function getOrder()
      */
     public function getOrderItem()
     {
-        if ($this->getItem() instanceof \Magento\Sales\Model\Order\Item) {
+        if ($this->getItem() instanceof OrderItem) {
             return $this->getItem();
         } else {
             return $this->getItem()->getOrderItem();
@@ -100,20 +105,20 @@ public function getOrderItem()
      */
     public function getItemOptions()
     {
-        $result = [];
+        $result = [[]];
         $options = $this->getOrderItem()->getProductOptions();
         if ($options) {
             if (isset($options['options'])) {
-                $result = array_merge($result, $options['options']);
+                $result[] = $options['options'];
             }
             if (isset($options['additional_options'])) {
-                $result = array_merge($result, $options['additional_options']);
+                $result[] = $options['additional_options'];
             }
             if (isset($options['attributes_info'])) {
-                $result = array_merge($result, $options['attributes_info']);
+                $result[] = $options['attributes_info'];
             }
         }
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
diff --git a/app/code/Magento/Sales/Model/Order/Pdf/Items/AbstractItems.php b/app/code/Magento/Sales/Model/Order/Pdf/Items/AbstractItems.php
index 422ff1746c9a6..29e011217ef20 100644
--- a/app/code/Magento/Sales/Model/Order/Pdf/Items/AbstractItems.php
+++ b/app/code/Magento/Sales/Model/Order/Pdf/Items/AbstractItems.php
@@ -6,41 +6,54 @@
 namespace Magento\Sales\Model\Order\Pdf\Items;
 
 use Magento\Framework\App\Filesystem\DirectoryList;
+use Magento\Framework\Data\Collection\AbstractDb;
+use Magento\Framework\DataObject;
+use Magento\Framework\Filesystem;
+use Magento\Framework\Filesystem\Directory\ReadInterface;
+use Magento\Framework\Filter\FilterManager;
+use Magento\Framework\Model\AbstractModel;
+use Magento\Framework\Model\Context;
+use Magento\Framework\Model\ResourceModel\AbstractResource;
+use Magento\Framework\Registry;
+use Magento\Sales\Model\Order;
+use Magento\Sales\Model\Order\Pdf\AbstractPdf;
+use Magento\Tax\Helper\Data as TaxHelper;
 
 /**
  * Sales Order Pdf Items renderer Abstract
  *
+ * phpcs:disable Magento2.Classes.AbstractApi
  * @api
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  * @since 100.0.2
  */
-abstract class AbstractItems extends \Magento\Framework\Model\AbstractModel
+abstract class AbstractItems extends AbstractModel
 {
     /**
      * Order model
      *
-     * @var \Magento\Sales\Model\Order
+     * @var Order
      */
     protected $_order;
 
     /**
      * Source model (invoice, shipment, creditmemo)
      *
-     * @var \Magento\Framework\Model\AbstractModel
+     * @var AbstractModel
      */
     protected $_source;
 
     /**
      * Item object
      *
-     * @var \Magento\Framework\DataObject
+     * @var DataObject
      */
     protected $_item;
 
     /**
      * Pdf object
      *
-     * @var \Magento\Sales\Model\Order\Pdf\AbstractPdf
+     * @var AbstractPdf
      */
     protected $_pdf;
 
@@ -54,38 +67,38 @@ abstract class AbstractItems extends \Magento\Framework\Model\AbstractModel
     /**
      * Tax data
      *
-     * @var \Magento\Tax\Helper\Data
+     * @var TaxHelper
      */
     protected $_taxData;
 
     /**
-     * @var \Magento\Framework\Filesystem\Directory\ReadInterface
+     * @var ReadInterface
      */
     protected $_rootDirectory;
 
     /**
-     * @var \Magento\Framework\Filter\FilterManager
+     * @var FilterManager
      */
     protected $filterManager;
 
     /**
-     * @param \Magento\Framework\Model\Context $context
-     * @param \Magento\Framework\Registry $registry
-     * @param \Magento\Tax\Helper\Data $taxData
-     * @param \Magento\Framework\Filesystem $filesystem ,
-     * @param \Magento\Framework\Filter\FilterManager $filterManager
-     * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
-     * @param \Magento\Framework\Data\Collection\AbstractDb $resourceCollection
+     * @param Context $context
+     * @param Registry $registry
+     * @param TaxHelper $taxData
+     * @param Filesystem $filesystem ,
+     * @param FilterManager $filterManager
+     * @param AbstractResource $resource
+     * @param AbstractDb $resourceCollection
      * @param array $data
      */
     public function __construct(
-        \Magento\Framework\Model\Context $context,
-        \Magento\Framework\Registry $registry,
-        \Magento\Tax\Helper\Data $taxData,
-        \Magento\Framework\Filesystem $filesystem,
-        \Magento\Framework\Filter\FilterManager $filterManager,
-        \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
-        \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
+        Context $context,
+        Registry $registry,
+        TaxHelper $taxData,
+        Filesystem $filesystem,
+        FilterManager $filterManager,
+        AbstractResource $resource = null,
+        AbstractDb $resourceCollection = null,
         array $data = []
     ) {
         $this->filterManager = $filterManager;
@@ -97,10 +110,10 @@ public function __construct(
     /**
      * Set order model
      *
-     * @param  \Magento\Sales\Model\Order $order
+     * @param  Order $order
      * @return $this
      */
-    public function setOrder(\Magento\Sales\Model\Order $order)
+    public function setOrder(Order $order)
     {
         $this->_order = $order;
         return $this;
@@ -109,10 +122,10 @@ public function setOrder(\Magento\Sales\Model\Order $order)
     /**
      * Set Source model
      *
-     * @param  \Magento\Framework\Model\AbstractModel $source
+     * @param  AbstractModel $source
      * @return $this
      */
-    public function setSource(\Magento\Framework\Model\AbstractModel $source)
+    public function setSource(AbstractModel $source)
     {
         $this->_source = $source;
         return $this;
@@ -121,10 +134,10 @@ public function setSource(\Magento\Framework\Model\AbstractModel $source)
     /**
      * Set item object
      *
-     * @param  \Magento\Framework\DataObject $item
+     * @param  DataObject $item
      * @return $this
      */
-    public function setItem(\Magento\Framework\DataObject $item)
+    public function setItem(DataObject $item)
     {
         $this->_item = $item;
         return $this;
@@ -133,10 +146,10 @@ public function setItem(\Magento\Framework\DataObject $item)
     /**
      * Set Pdf model
      *
-     * @param  \Magento\Sales\Model\Order\Pdf\AbstractPdf $pdf
+     * @param  AbstractPdf $pdf
      * @return $this
      */
-    public function setPdf(\Magento\Sales\Model\Order\Pdf\AbstractPdf $pdf)
+    public function setPdf(AbstractPdf $pdf)
     {
         $this->_pdf = $pdf;
         return $this;
@@ -313,20 +326,20 @@ public function getItemPricesForDisplay()
      */
     public function getItemOptions()
     {
-        $result = [];
+        $result = [[]];
         $options = $this->getItem()->getOrderItem()->getProductOptions();
         if ($options) {
             if (isset($options['options'])) {
-                $result = array_merge($result, $options['options']);
+                $result[] = $options['options'];
             }
             if (isset($options['additional_options'])) {
-                $result = array_merge($result, $options['additional_options']);
+                $result[] = $options['additional_options'];
             }
             if (isset($options['attributes_info'])) {
-                $result = array_merge($result, $options['attributes_info']);
+                $result[] = $options['attributes_info'];
             }
         }
-        return $result;
+        return array_merge(...$result);
     }
 
     /**
diff --git a/app/code/Magento/Sales/Model/ResourceModel/Provider/NotSyncedDataProvider.php b/app/code/Magento/Sales/Model/ResourceModel/Provider/NotSyncedDataProvider.php
index aa2ac20f8f55c..645e411b80b67 100644
--- a/app/code/Magento/Sales/Model/ResourceModel/Provider/NotSyncedDataProvider.php
+++ b/app/code/Magento/Sales/Model/ResourceModel/Provider/NotSyncedDataProvider.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Sales\Model\ResourceModel\Provider;
 
 use Magento\Framework\ObjectManager\TMapFactory;
@@ -21,10 +22,8 @@ class NotSyncedDataProvider implements NotSyncedDataProviderInterface
      * @param TMapFactory $tmapFactory
      * @param array $providers
      */
-    public function __construct(
-        TMapFactory $tmapFactory,
-        array $providers = []
-    ) {
+    public function __construct(TMapFactory $tmapFactory, array $providers = [])
+    {
         $this->providers = $tmapFactory->create(
             [
                 'array' => $providers,
@@ -38,11 +37,11 @@ public function __construct(
      */
     public function getIds($mainTableName, $gridTableName)
     {
-        $result = [];
+        $result = [[]];
         foreach ($this->providers as $provider) {
-            $result = array_merge($result, $provider->getIds($mainTableName, $gridTableName));
+            $result[] = $provider->getIds($mainTableName, $gridTableName);
         }
 
-        return array_unique($result);
+        return array_unique(array_merge(...$result));
     }
 }
diff --git a/app/code/Magento/Search/Model/Autocomplete.php b/app/code/Magento/Search/Model/Autocomplete.php
index b50cd330d5d95..45957e8795744 100644
--- a/app/code/Magento/Search/Model/Autocomplete.php
+++ b/app/code/Magento/Search/Model/Autocomplete.php
@@ -5,6 +5,9 @@
  */
 namespace Magento\Search\Model;
 
+/**
+ * Provides list of Autocomplete items
+ */
 class Autocomplete implements AutocompleteInterface
 {
     /**
@@ -23,15 +26,15 @@ public function __construct(
     }
 
     /**
-     * {@inheritdoc}
+     * @inheritdoc
      */
     public function getItems()
     {
-        $data = [];
+        $data = [[]];
         foreach ($this->dataProviders as $dataProvider) {
-            $data = array_merge($data, $dataProvider->getItems());
+            $data[] = $dataProvider->getItems();
         }
 
-        return $data;
+        return array_merge(...$data);
     }
 }
diff --git a/app/code/Magento/Search/Model/SynonymGroupRepository.php b/app/code/Magento/Search/Model/SynonymGroupRepository.php
index 75d7049afd949..dbc2b66b1f047 100644
--- a/app/code/Magento/Search/Model/SynonymGroupRepository.php
+++ b/app/code/Magento/Search/Model/SynonymGroupRepository.php
@@ -7,6 +7,7 @@
 namespace Magento\Search\Model;
 
 use Magento\Framework\Exception\CouldNotDeleteException;
+use Magento\Framework\Phrase;
 use Magento\Search\Api\Data\SynonymGroupInterface;
 use Magento\Search\Api\SynonymGroupRepositoryInterface;
 use Magento\Search\Model\ResourceModel\SynonymGroup as SynonymGroupResourceModel;
@@ -37,7 +38,7 @@ class SynonymGroupRepository implements SynonymGroupRepositoryInterface
      * @param SynonymGroupResourceModel $resourceModel
      */
     public function __construct(
-        \Magento\Search\Model\SynonymGroupFactory $synonymGroupFactory,
+        SynonymGroupFactory $synonymGroupFactory,
         SynonymGroupResourceModel $resourceModel
     ) {
         $this->synonymGroupFactory = $synonymGroupFactory;
@@ -64,8 +65,8 @@ public function save(SynonymGroupInterface $synonymGroup, $errorOnMergeConflict
      * Deletes a synonym group
      *
      * @param SynonymGroupInterface $synonymGroup
-     * @throws CouldNotDeleteException
      * @return bool
+     * @throws CouldNotDeleteException
      */
     public function delete(SynonymGroupInterface $synonymGroup)
     {
@@ -149,17 +150,17 @@ private function create(SynonymGroupInterface $synonymGroup, $errorOnMergeConfli
      */
     private function merge(SynonymGroupInterface $synonymGroupToMerge, array $matchingGroupIds)
     {
-        $mergedSynonyms = [];
+        $mergedSynonyms = [[]];
         foreach ($matchingGroupIds as $groupId) {
             /** @var SynonymGroup $synonymGroupModel */
             $synonymGroupModel = $this->synonymGroupFactory->create();
             $synonymGroupModel->load($groupId);
-            $mergedSynonyms = array_merge($mergedSynonyms, explode(',', $synonymGroupModel->getSynonymGroup()));
+            $mergedSynonyms[] = explode(',', $synonymGroupModel->getSynonymGroup());
             $synonymGroupModel->delete();
         }
-        $mergedSynonyms = array_merge($mergedSynonyms, explode(',', $synonymGroupToMerge->getSynonymGroup()));
-        $mergedSynonyms = array_unique($mergedSynonyms);
-        return $mergedSynonyms;
+        $mergedSynonyms[] = explode(',', $synonymGroupToMerge->getSynonymGroup());
+
+        return array_unique(array_merge(...$mergedSynonyms));
     }
 
     /**
@@ -222,7 +223,7 @@ private function update(
      * Gets merge conflict exception message
      *
      * @param string[] $matchingSynonymGroups
-     * @return \Magento\Framework\Phrase
+     * @return Phrase
      */
     private function getExceptionMessage($matchingSynonymGroups)
     {
diff --git a/app/code/Magento/Tax/Model/ResourceModel/Calculation/Rate/Collection.php b/app/code/Magento/Tax/Model/ResourceModel/Calculation/Rate/Collection.php
index 9b344f6e96f65..7863b70f6626a 100644
--- a/app/code/Magento/Tax/Model/ResourceModel/Calculation/Rate/Collection.php
+++ b/app/code/Magento/Tax/Model/ResourceModel/Calculation/Rate/Collection.php
@@ -7,9 +7,25 @@
 /**
  * Tax rate collection
  */
+
 namespace Magento\Tax\Model\ResourceModel\Calculation\Rate;
 
-class Collection extends \Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection
+use Magento\Framework\Data\Collection\Db\FetchStrategyInterface;
+use Magento\Framework\Data\Collection\EntityFactory;
+use Magento\Framework\DB\Adapter\AdapterInterface;
+use Magento\Framework\Event\ManagerInterface;
+use Magento\Framework\Model\ResourceModel\Db\AbstractDb;
+use Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;
+use Magento\Store\Model\Store;
+use Magento\Store\Model\StoreManagerInterface;
+use Magento\Tax\Model\Calculation\Rate;
+use Magento\Tax\Model\ResourceModel\Calculation\Rate as RateResourceModel;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Collection of Calculation Rates
+ */
+class Collection extends AbstractCollection
 {
     /**
      * Value of fetched from DB of rules per cycle
@@ -17,27 +33,27 @@ class Collection extends \Magento\Framework\Model\ResourceModel\Db\Collection\Ab
     const TAX_RULES_CHUNK_SIZE = 1000;
 
     /**
-     * @var \Magento\Store\Model\StoreManagerInterface
+     * @var StoreManagerInterface
      */
     protected $_storeManager;
 
     /**
-     * @param \Magento\Framework\Data\Collection\EntityFactory $entityFactory
-     * @param \Psr\Log\LoggerInterface $logger
-     * @param \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy
-     * @param \Magento\Framework\Event\ManagerInterface $eventManager
-     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
+     * @param EntityFactory $entityFactory
+     * @param LoggerInterface $logger
+     * @param FetchStrategyInterface $fetchStrategy
+     * @param ManagerInterface $eventManager
+     * @param StoreManagerInterface $storeManager
      * @param mixed $connection
-     * @param \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource
+     * @param AbstractDb $resource
      */
     public function __construct(
-        \Magento\Framework\Data\Collection\EntityFactory $entityFactory,
-        \Psr\Log\LoggerInterface $logger,
-        \Magento\Framework\Data\Collection\Db\FetchStrategyInterface $fetchStrategy,
-        \Magento\Framework\Event\ManagerInterface $eventManager,
-        \Magento\Store\Model\StoreManagerInterface $storeManager,
-        \Magento\Framework\DB\Adapter\AdapterInterface $connection = null,
-        \Magento\Framework\Model\ResourceModel\Db\AbstractDb $resource = null
+        EntityFactory $entityFactory,
+        LoggerInterface $logger,
+        FetchStrategyInterface $fetchStrategy,
+        ManagerInterface $eventManager,
+        StoreManagerInterface $storeManager,
+        AdapterInterface $connection = null,
+        AbstractDb $resource = null
     ) {
         $this->_storeManager = $storeManager;
         parent::__construct($entityFactory, $logger, $fetchStrategy, $eventManager, $connection, $resource);
@@ -50,10 +66,7 @@ public function __construct(
      */
     protected function _construct()
     {
-        $this->_init(
-            \Magento\Tax\Model\Calculation\Rate::class,
-            \Magento\Tax\Model\ResourceModel\Calculation\Rate::class
-        );
+        $this->_init(Rate::class, RateResourceModel::class);
     }
 
     /**
@@ -90,7 +103,7 @@ public function joinRegionTable()
     /**
      * Join rate title for specified store
      *
-     * @param \Magento\Store\Model\Store|string|int $store
+     * @param Store|string|int $store
      * @return $this
      */
     public function joinTitle($store = null)
@@ -170,12 +183,10 @@ public function toOptionHash()
     }
 
     /**
-     * Convert items array to hash for select options
-     * using fetchItem method
-     *
-     * @see fetchItem()
+     * Convert items array to hash for select options using fetchItem method
      *
      * @return array
+     * @see fetchItem()
      */
     public function toOptionHashOptimized()
     {
@@ -195,7 +206,7 @@ public function getOptionRates()
     {
         $size = self::TAX_RULES_CHUNK_SIZE;
         $page = 1;
-        $rates = [];
+        $rates = [[]];
         do {
             $offset = $size * ($page - 1);
             $this->getSelect()->reset();
@@ -206,11 +217,11 @@ public function getOptionRates()
                 )
                 ->limit($size, $offset);
 
-            $rates = array_merge($rates, $this->toOptionArray());
+            $rates[] = $this->toOptionArray();
             $this->clear();
             $page++;
         } while ($this->getSize() > $offset);
 
-        return $rates;
+        return array_merge(...$rates);
     }
 }
diff --git a/app/code/Magento/Weee/Model/Total/Quote/Weee.php b/app/code/Magento/Weee/Model/Total/Quote/Weee.php
index 96a40c9533f8f..449c6cd688668 100644
--- a/app/code/Magento/Weee/Model/Total/Quote/Weee.php
+++ b/app/code/Magento/Weee/Model/Total/Quote/Weee.php
@@ -7,10 +7,19 @@
 namespace Magento\Weee\Model\Total\Quote;
 
 use Magento\Framework\Pricing\PriceCurrencyInterface;
+use Magento\Quote\Api\Data\ShippingAssignmentInterface;
+use Magento\Quote\Model\Quote;
+use Magento\Quote\Model\Quote\Address;
+use Magento\Quote\Model\Quote\Address\Total;
 use Magento\Quote\Model\Quote\Address\Total\AbstractTotal;
+use Magento\Quote\Model\Quote\Item\AbstractItem;
 use Magento\Store\Model\Store;
 use Magento\Tax\Model\Sales\Total\Quote\CommonTaxCollector;
+use Magento\Weee\Helper\Data as WeeHelper;
 
+/**
+ * Calculate totals for Quote
+ */
 class Weee extends AbstractTotal
 {
     /**
@@ -23,7 +32,7 @@ class Weee extends AbstractTotal
     const ITEM_TYPE = 'weee';
 
     /**
-     * @var \Magento\Weee\Helper\Data
+     * @var WeeHelper
      */
     protected $weeeData;
 
@@ -66,11 +75,11 @@ class Weee extends AbstractTotal
     protected $priceCurrency;
 
     /**
-     * @param \Magento\Weee\Helper\Data $weeeData
+     * @param WeeHelper $weeeData
      * @param PriceCurrencyInterface $priceCurrency
      */
     public function __construct(
-        \Magento\Weee\Helper\Data $weeeData,
+        WeeHelper $weeeData,
         PriceCurrencyInterface $priceCurrency
     ) {
         $this->priceCurrency = $priceCurrency;
@@ -82,15 +91,15 @@ public function __construct(
     /**
      * Collect Weee amounts for the quote / order
      *
-     * @param \Magento\Quote\Model\Quote $quote
-     * @param \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment
-     * @param \Magento\Quote\Model\Quote\Address\Total $total
+     * @param Quote $quote
+     * @param ShippingAssignmentInterface $shippingAssignment
+     * @param Total $total
      * @return $this
      */
     public function collect(
-        \Magento\Quote\Model\Quote $quote,
-        \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,
-        \Magento\Quote\Model\Quote\Address\Total $total
+        Quote $quote,
+        ShippingAssignmentInterface $shippingAssignment,
+        Total $total
     ) {
         AbstractTotal::collect($quote, $shippingAssignment, $total);
         $this->_store = $quote->getStore();
@@ -130,16 +139,16 @@ public function collect(
     /**
      * Calculate item fixed tax and prepare information for discount and regular taxation
      *
-     * @param   \Magento\Quote\Model\Quote\Address $address
-     * @param   \Magento\Quote\Model\Quote\Address\Total $total
-     * @param   \Magento\Quote\Model\Quote\Item\AbstractItem $item
-     * @return  void|$this
+     * @param Address $address
+     * @param Total $total
+     * @param AbstractItem $item
+     * @return void|$this
      * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
      * @SuppressWarnings(PHPMD.UnusedLocalVariable)
      */
     protected function process(
-        \Magento\Quote\Model\Quote\Address $address,
-        \Magento\Quote\Model\Quote\Address\Total $total,
+        Address $address,
+        Total $total,
         $item
     ) {
         $attributes = $this->weeeData->getProductWeeeAttributes(
@@ -248,12 +257,12 @@ protected function process(
     /**
      * Process row amount based on FPT total amount configuration setting
      *
-     * @param   \Magento\Quote\Model\Quote\Address\Total $total
-     * @param   float $rowValueExclTax
-     * @param   float $baseRowValueExclTax
-     * @param   float $rowValueInclTax
-     * @param   float $baseRowValueInclTax
-     * @return  $this
+     * @param Total $total
+     * @param float $rowValueExclTax
+     * @param float $baseRowValueExclTax
+     * @param float $rowValueInclTax
+     * @param float $baseRowValueInclTax
+     * @return $this
      */
     protected function processTotalAmount(
         $total,
@@ -279,8 +288,7 @@ protected function processTotalAmount(
     }
 
     /**
-     * Increment and return counter. This function is intended to be used to generate temporary
-     * id for an item.
+     * Increment and return counter. This function is intended to be used to generate temporary id for an item.
      *
      * @return int
      */
@@ -292,25 +300,26 @@ protected function getNextIncrement()
     /**
      * Recalculate parent item amounts based on children results
      *
-     * @param   \Magento\Quote\Model\Quote\Item\AbstractItem $item
-     * @return  void
-     *
+     * @param AbstractItem $item
+     * @return void
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
-    protected function recalculateParent(\Magento\Quote\Model\Quote\Item\AbstractItem $item)
+    protected function recalculateParent(AbstractItem $item)
     {
-        $associatedTaxables = [];
+        $associatedTaxables = [[]];
         foreach ($item->getChildren() as $child) {
-            $associatedTaxables = array_merge($associatedTaxables, $child->getAssociatedTaxables());
+            $associatedTaxables[] = $child->getAssociatedTaxables();
         }
-        $item->setAssociatedTaxables($associatedTaxables);
+        $item->setAssociatedTaxables(
+            array_unique(array_merge(...$associatedTaxables))
+        );
     }
 
     /**
      * Reset information about Tax and Wee on FPT for shopping cart item
      *
-     * @param   \Magento\Quote\Model\Quote\Item\AbstractItem $item
-     * @return  void
+     * @param AbstractItem $item
+     * @return void
      */
     protected function resetItemData($item)
     {
@@ -332,24 +341,24 @@ protected function resetItemData($item)
     }
 
     /**
-     * @param \Magento\Quote\Model\Quote $quote
-     * @param \Magento\Quote\Model\Quote\Address\Total $total
+     * Return `null` instead of original empty array
+     *
+     * @param Quote $quote
+     * @param Total $total
      * @return array|null
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
-    public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)
+    public function fetch(Quote $quote, Total $total)
     {
         return null;
     }
 
     /**
-     * Process model configuration array.
-     * This method can be used for changing totals collect sort order
-     *
-     * @param   array $config
-     * @param   Store $store
-     * @return  array
+     * Process model configuration array. This method can be used for changing totals collect sort order
      *
+     * @param array $config
+     * @param Store $store
+     * @return array
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
     public function processConfigArray($config, $store)
@@ -360,7 +369,7 @@ public function processConfigArray($config, $store)
     /**
      * No aggregated label for fixed product tax
      *
-     * TODO: fix
+     * @TODO: fix
      * @return string
      */
     public function getLabel()