Backport #[DelayedTargetValidation] to 8.3 + 8.4

[kkmuffme]

Description

class A {
    public const XYZ = 'hello';
	
    public string $bar = '';
    
    public function __construct($a) {
        $this->bar = $a;
    }
}

class B extends A {
    #[DelayedTargetValidation]
    #[Override]
    public const XYZ = 'world';
	
    public string $bar = 'asd';

    #[DelayedTargetValidation]
    #[Deprecated]
    public string $ddd = 'asd';
}

$b = new B('a');

echo $b->ddd;

This code works in PHP <= 8.2 and >= 8.5
(the Deprecated case in <=8.3 and >= 8.5)
since #[DelayedTargetValidation] was only added in 8.5 - creating a gap of 8.3 and 8.4

This means that 2 important (since they're among the first that were implemented) built-in attributes can't be used for many use cases until at least 5 more years (looking at how common PHP 7.4 still is, despite it being unmaintained since years, just like how it was with PHP 5.6 too back then)

I'm not familiar if possible at all, but would it theoretically be possible to backport #[DelayedTargetValidation] to also work in 8.3 + 8.4?

[DanielEScherzer]

Yeah, I had thought about proposing this when I proposed the RFC, but wasn't sure
@php/release-managers-84 @php/release-managers-83 any thoughts before bringing this up on internals?

[DanielEScherzer]

Also CC @remicollet since DelayedTargetValidation changed the ABI - I'm not sure if any other extensions register attributes with custom validators, but if they do they would break if not updated

[NattyNarwhal]

Not inherently opposed as long as it isn't breaking, but distributions with older PHP doing their backported fixes to a specific patch release could be a problem. If i.e. Debian won't include it, applications may still be hesitant.