⏫ Forwardport of #12107 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12107.patch (created by @p-bystritsky) based on commit(s):
  1. ca071c0

Fixed GitHub Issues in 2.3-develop branch:
  - #11880: Magento 2.1.9 Configurable::getUsedProducts returns a different array after product collections is cached (reported by @apedicdev)

Author: magento-engcom-team

app/code/Magento/ConfigurableProduct/Model/Product/Type/Configurable.php

@@ -1296,7 +1296,7 @@ private function loadUsedProducts(\Magento\Catalog\Model\Product $product, $cach
                 if ($salableOnly) {
                     $collection = $this->salableProcessor->process($collection);
                 }
-                $usedProducts = $collection->getItems();
+                $usedProducts = array_values($collection->getItems());
                 $this->saveUsedProductsCacheData($product, $usedProducts, $cacheKey);
             }
             $product->setData($dataFieldName, $usedProducts);

dev/tests/integration/testsuite/Magento/ConfigurableProduct/Model/Product/Type/ConfigurableTest.php

@@ -235,6 +235,26 @@ public function testGetUsedProducts()
         }
     }
 
+    /**
+     * Test getUsedProducts returns array with same indexes regardless collections was cache or not.
+     *
+     * @magentoAppIsolation enabled
+     * @magentoDataFixture Magento/ConfigurableProduct/_files/product_configurable.php
+     */
+    public function testGetUsedProductsCached()
+    {
+        /** @var  \Magento\Framework\App\Cache\StateInterface $cacheState */
+        $cacheState = Bootstrap::getObjectManager()->get(\Magento\Framework\App\Cache\StateInterface::class);
+        $cacheState->setEnabled(\Magento\Framework\App\Cache\Type\Collection::TYPE_IDENTIFIER, true);
+
+        $products = $this->getUsedProducts();
+        $productsCached = $this->getUsedProducts();
+        self::assertEquals(
+            array_keys($products),
+            array_keys($productsCached)
+        );
+    }
+
     public function testGetUsedProductCollection()
     {
         $this->assertInstanceOf(
@@ -570,4 +590,14 @@ protected function _prepareForCart()
 
         return $product;
     }
+
+    /**
+     * @return ProductInterface[]
+     */
+    protected function getUsedProducts()
+    {
+        $product = Bootstrap::getObjectManager()->create(Product::class);
+        $product->load(1);
+        return $this->model->getUsedProducts($product);
+    }
 }