[Backport] Fix proxy generation return type

lib/internal/Magento/Framework/ObjectManager/Code/Generator/Proxy.php

@@ -165,6 +165,7 @@ protected function _getMethodInfo(\ReflectionMethod $method)
             'parameters' => $parameters,
             'body' => $this->_getMethodBody($method->getName(), $parameterNames),
             'docblock' => ['shortDescription' => '{@inheritdoc}'],
+            'returnType' => $method->getReturnType(),
         ];
 
         return $methodInfo;

lib/internal/Magento/Framework/ObjectManager/Test/Unit/Code/Generator/_files/Sample.php

@@ -16,6 +16,11 @@ class Sample
      */
     protected $messages = [];
 
+    /**
+     * @var array
+     */
+    private $config = [];
+
     /**
      * @param array $messages
      */
@@ -31,4 +36,20 @@ public function getMessages()
     {
         return $this->messages;
     }
+
+    /**
+     * @param array $config
+     */
+    public function setConfig(array $config)
+    {
+        $this->config = $config;
+    }
+
+    /**
+     * @return array
+     */
+    public function getConfig(): array
+    {
+        return $this->config;
+    }
 }

lib/internal/Magento/Framework/ObjectManager/Test/Unit/Code/Generator/_files/SampleProxy.txt

@@ -101,4 +101,20 @@ class Sample_Proxy extends \Magento\Framework\ObjectManager\Code\Generator\Sampl
     {
         return $this->_getSubject()->getMessages();
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function setConfig(array $config)
+    {
+        return $this->_getSubject()->setConfig($config);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getConfig() : array
+    {
+        return $this->_getSubject()->getConfig();
+    }
 }