Description:
------------
Hi,

Standard PHP 7.2.10 installation on Windows 7x64.


Test script:
---------------
<?php

declare(strict_types=1);

namespace ns1 {
    class abc
    {

    }
};

namespace ns2 {
    interface interfaceXyz
    {
        public function getMe(): \ns1\abc;
    }
};

namespace ns3 {
    class def extends \ns1\abc
    {
        function checkInstance()
        {
            return ($this instanceof \ns1\abc)
                ? '\ns3\def is an instance of \ns1\abc'
                : '\ns3\def is different of \ns1\abc';
        }
    }

    echo (new def)->checkInstance();     // result = "\ns3\def is an instance of \ns1\abc"
};

namespace ns4 {
    class ghi implements \ns2\interfaceXyz
    {
        public function getMe(): \ns3\def
        {
            // here : \ns3\def (derivated from \ns1\abc ) is an instance of \ns1\abc but is not compliant with the interface that expect an \ns1\abc instance !!!
            return 'hello world';
        }
    }

    echo (new ghi)->getMe();

}

Expected result:
----------------
Hello world

Actual result:
--------------
\ns3\def is an instance of \ns1\abc

Fatal error: Declaration of ns4\ghi::getMe(): ns3\def must be compatible with ns2\interfaceXyz::getMe(): ns1\abc