polymorphicError

<?php
    namespace test;
    abstract class A
    {
        //public function foo()
        protected function foo()
        {
            echo 'AA';
        }
    }

    class B extends A
    {
        public function foo()
        {
            echo 'BB';
        }
    }

    function bar(A $x)
    {
        $x->foo();
    }

$b = new B;
    bar($b);// BB expected, but if "foo" access modifier change to protected in parent class then
            // PhpStorm suppose an error(underlined with a red wavy line),
            // but, naturally,  runtime error will not occurred