Undefined field of derived stdClass shows warning

class TestClass extends stdClass
{
   public function __construct() {
   }
}


function test() {
    $x = new stdClass();
    $y = new TestClass();
    print $x->test . $y->test;
}
 
$x->test shows no warning.

$y->test shows a warning

Message shows:

Referenced field is not      found in subject class.
Note: Check is not performed on objects of type      "stdClass" or derived.

Could someone explain?

this is in phpstorm 3.0

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