<?php

class AAA{}
class BBB extends AAA{}

class X<T is AAA>{
    protected T $foo;
    function __construct(T $foo){
        $this->foo = $foo;
    }
    function set(T $foo){
        $this->foo = $foo;
    }
}

class Y<T is BBB> extends X<T>{
    function __construct(T $foo){
        parent::__construct($foo);
    }
}