Merge branch '5.1' into 5.2

* 5.1:
  Changed private static array-properties to const

Author: nicolas-grekas

PropertyAccessor.php

@@ -76,7 +76,7 @@ class PropertyAccessor implements PropertyAccessorInterface
     private $writeInfoExtractor;
     private $readPropertyCache = [];
     private $writePropertyCache = [];
-    private static $resultProto = [self::VALUE => null];
+    private const RESULT_PROTO = [self::VALUE => null];
 
     /**
      * Should not be used by application code. Use
@@ -379,7 +379,7 @@ private function readIndex(array $zval, $index): array
             throw new NoSuchIndexException(sprintf('Cannot read index "%s" from object of type "%s" because it doesn\'t implement \ArrayAccess.', $index, get_debug_type($zval[self::VALUE])));
         }
 
-        $result = self::$resultProto;
+        $result = self::RESULT_PROTO;
 
         if (isset($zval[self::VALUE][$index])) {
             $result[self::VALUE] = $zval[self::VALUE][$index];
@@ -407,7 +407,7 @@ private function readProperty(array $zval, string $property, bool $ignoreInvalid
             throw new NoSuchPropertyException(sprintf('Cannot read property "%s" from an array. Maybe you intended to write the property path as "[%1$s]" instead.', $property));
         }
 
-        $result = self::$resultProto;
+        $result = self::RESULT_PROTO;
         $object = $zval[self::VALUE];
         $class = \get_class($object);
         $access = $this->getReadInfo($class, $property);