Initialisation of readonly properties by reference does not work (e.g. matches of preg_match)

[snk-spo]

Description

The following code:

https://3v4l.org/DdVMXM#v8.3.3

<?php

class A
{
    private readonly array $matches;
    
    public function __construct()
    {
        \preg_match('/t/', '', $this->matches);
    }
}

new A();

Resulted in this output:

Uncaught Error: Cannot indirectly modify readonly property A::$matches

But I expected this output instead:
The same-scope uninitialised readonly property is initialised by preg_match matches. Such by-reference initialisation of core method arguments works for local variables and untyped writable properties. I did not find anything in the documentation regarding readonly in combination with by-reference.
Even empty array (no matches) initialisation (which definitley would be a write-once operation) triggers the error.
Is this intended, a bug or an overlooked case?

If it actually is intended shouldn't it throw the following Error instead, however?

Uncaught Error: Cannot access uninitialized non-nullable property A::$matches by reference

PHP Version

PHP 8.3

Operating System

Ubuntu 23.10

[damianwadley]

I believe this is intended behavior: you can't "modify" read-only properties through a reference. Only through direct assignment.

The docs aren't really clear about this, but it is alluded to as a type of non-"plain" assignment.

https://www.php.net/manual/en/language.oop5.properties.php#language.oop5.properties.readonly-properties

Modifications are not necessarily plain assignments, all of the following will also result in an Error exception:

$ref =& $test->i;

byRef($test->i);

That code was lifted straight from the RFC so it wasn't quite intended to be suitable for posting in the docs...

[github-actions]

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[damianwadley]

Switching this to a doc bug because I believe the code used on that page, while fine for an RFC, needs to be cleaned up some (like with added comments) to make it more suitable for general documentation.