Typed and untyped object properties behave differently in PHP when their declaration doesn't include an initial value:

<?php

class C {
    public $untyped;
    public string $typed;
}

$c = new C;
isset( $c->untyped ); // false
isset( $c->typed ); // false
var_dump( $c->untyped ); // NULL
var_dump( $c->typed ); // Fatal error: Typed property C::$typed must not be accessed before initialization

(3v4l)
so it's often important to check if a typed property has been initialized before accessing it. Phan however seems to think isset( $c->typed ) is always true (as seen above, this is not the case), and warns about it.

Upstream task is phan/phan#4720.