[RFC] Prevent instantiation and cloning of __PHP_Incomplete_Class

Hi

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.

I don't find the motivation particularly compelling. Yes, manually
instantiating the class is not particularly useful, but it's also not
harmful. Similarly for cloning: If the cloning operation works correctly
(i.e. it returning a new object with the same state), then it is just
behaving according to expectations.

On the contrary, I would like to see fewer behavioral differences
between internal classes and userland classes. A class that can only be
instantiated by the engine without there being a good engine-related
reason is yet another special case that users need to learn about.

Best regards
Tim Düsterhus

PS: Unserialization for unknown classes should just throw an Exception.