Merge branch '6.4' into 7.3

* 6.4:
  [VarExporter] Fix serializing classes with __serialize() returning unprefixed private properties
  Remove direct access to internal properties cross-components

Author: nicolas-grekas

Tests/Validator/Constraints/TwigTest.php

@@ -27,15 +27,15 @@ public function testAttributes()
         $loader = new AttributeLoader();
         self::assertTrue($loader->loadClassMetadata($metadata));
 
-        [$bConstraint] = $metadata->properties['b']->getConstraints();
+        [$bConstraint] = $metadata->getPropertyMetadata('b')[0]->getConstraints();
         self::assertSame('myMessage', $bConstraint->message);
         self::assertSame(['Default', 'TwigDummy'], $bConstraint->groups);
 
-        [$cConstraint] = $metadata->properties['c']->getConstraints();
+        [$cConstraint] = $metadata->getPropertyMetadata('c')[0]->getConstraints();
         self::assertSame(['my_group'], $cConstraint->groups);
         self::assertSame('some attached data', $cConstraint->payload);
 
-        [$dConstraint] = $metadata->properties['d']->getConstraints();
+        [$dConstraint] = $metadata->getPropertyMetadata('d')[0]->getConstraints();
         self::assertFalse($dConstraint->skipDeprecations);
     }
 }