[RFC] Prevent instantiation and cloning of __PHP_Incomplete_Class

Hi internals,

I would like to propose a new RFC titled “Prevent instantiation and cloning of __PHP_Incomplete_Class”.

RFC: https://wiki.php.net/rfc/deprecate-incomplete-class-instantiation
Implementation PR: https://github.com/php/php-src/pull/21325

This RFC proposes to disallow direct instantiation and cloning of the internal class __PHP_Incomplete_Class.

This change introduces a backward compatibility break. The expected impact is small, as __PHP_Incomplete_Class is an internal class and not intended for direct use. Only code that explicitly instantiates or clones it would be affected.

I have two immediate questions for discussion:

1. Should we target PHP 8.6 or PHP 9.x? Given the limited scope of the break, I am currently leaning towards 8.6, though this is a (small) BC break. If we lean towards 9.x, I propose to add deprecations in 8.6.

2. Should instantiation and cloning be voted on separately, or is a single combined vote acceptable?

Feedback is very welcome.

Regards,
Jordi Kroon