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Redeclaring protected properties on children creates a liskov violation #19406
Description
Description
The following code:
<?php
class P {
protected $v;
}
class C extends P {
public function test(P $i) {
return $i->v;
}
}
class C2 extends P {
protected $v;
}
$c = new C;
$c2 = new C2;
$c->test($c2);Resulted in this output:
PHP Fatal error: Uncaught Error: Cannot access protected property C2::$v
But I expected this output instead:
After a discussion on the mailing list everyone seemed to agree it's a bug, and I believe fixing it wouldn't break any pre-existing code. Properties were invariant up to 8.4, and property hook variance is well defined.
PHP Version
PHP 8.4.10 (cli) (built: Jul 3 2025 12:35:27) (NTS)
Copyright (c) The PHP Group
Built by Debian
Zend Engine v4.4.10, Copyright (c) Zend Technologies
with Zend OPcache v8.4.10, Copyright (c), by Zend Technologies
with Xdebug v3.4.3, Copyright (c) 2002-2025, by Derick Rethans
Operating System
No response
Activity
alecpl commentedon Aug 10, 2025
I don't really know Liskov, but you're accessing a protected property from outside. The fact that C extends P has nothing to do with this. How this is a bug?
alecpl commentedon Aug 10, 2025
OK, I saw the attached PR. I think I understand now. And I don't agree.
iluuu1994 commentedon Aug 15, 2025
@alecpl The properties are related because of their shared parent property. Methods work the same way. You can call protected methods that are overridden in sibling classes.
@jnvsor Can you try PHP 8.4.12 RC1? It should be solved there by #19046. It should be released in ~2 weeks.
jnvsor commentedon Aug 15, 2025
Thanks @iluuu1994 -- Tested with
docker run -it php:8.4.12RC1-zts-bookworm php -aand it doesn't throw an error any moreiluuu1994 commentedon Aug 15, 2025
@jnvsor Thanks for testing!