Cannot apply #[\Attribute] to abstract class on PHP 8.5

[nicolas-grekas]

Description

This worked on 8.4 and before, and doesn't anymore on 8.5:

<?php


#[\Attribute(\Attribute::TARGET_CLASS)]
abstract class Base
{
}

#[\Attribute(\Attribute::TARGET_CLASS)]
class Child extends Base
{
}

#[Child]
class A
{
}
$r = new \ReflectionClass('A');

var_dump($r->getAttributes('Base', ReflectionAttribute::IS_INSTANCEOF));

Resulted in this output:

Fatal error: Cannot apply #[\Attribute] to abstract class Base in /in/isSn6 on line 5

But I expected this output instead:

array(1) {
  [0]=>
  object(ReflectionAttribute)#2 (1) {
    ["name"]=>
    string(5) "Child"
  }
}

PHP Version

PHP 8.5

Operating System

No response

[DanielEScherzer]

See #19153 - I guess I didn't see the use case of IS_INSTANCEOF
Cross link symfony/symfony@0197b50#r165128320

But, why does the base class need to be marked as an attribute? As far as I understand, the benefit of marking userland classes as attributes is only to be able to use ReflectionAttribute::newInstance() - and that doesn't work with abstract classes anyway

[nicolas-grekas]

I need to investigate why I had a failure when the attribute was missing on the abstract class.
Looks like it's not the case in simple tests like https://3v4l.org/Flqql
Let me get back on this ASAP.

[nicolas-grekas]

I figured we had wrong logic on our side, see symfony/symfony#61649

I'll let you decide if this is worth fixing anyway. I've no data about the BC-breaking impact this change could have on 8.5...

[iluuu1994]

@nicolas-grekas Thank you! @DanielEScherzer Please decide whether you want to relax this for abstract classes and potentially interfaces. Otherwise, please close the issue.

[DanielEScherzer]

I searched for things marked as attributes on GitHub (/#\[\\?Attribute/ lang:php -is:fork -is:archived) and there are 36k hits so I'm not going to manually analyze the BC break impact, but given that

• the only benefit of #[\Attribute] is that ReflectionAttribute::newInstance() doesn't complain about classes not marked as attributes
• you can use non-attribute classes as attributes and as long as you don't call ::newInstance() nothing breaks
• if you were to call ::newInstance() with a trait/abstract class/interface/enum you would have gotten an error in 8.4 anyway from the inability to construct a new instance

I think any BC break is

• easy to mitigate (remove the #[\Attribute] from the trait/abstract class/interface/enum)
• with a backwards compatible mitigation (no harm in removing the attribute on 8.4 or below, it wasn't really working)
• can also be mitigated with #[\DelayedTargetValidation]

this can just be closed