Allow extending readonly classes by non-readonly ones

[kocsismate]

Based on the discussion of https://externals.io/message/118554

[derickr]

I don't think that readonly classes should be inheritable by non-readonly classes, as this would surely invalidate LSP compliance?

[deleugpn]

@derickr LSP is mostly about external factors about a class while readonly is about internals of a class. If one type-hints a readonly class, one shouldn't be doing $class->foo = 'bar'; anyway. Doing so means the user of the class is breaking LSP and relying on non-readonly children. I can't find any LSP reasoning that this should be prohibited.

Overall, I am against this change and I think it shouldn't be allowed, but I can't find any justification for why I feel this way.

[bdsl]

Agreeing with @deleugpn I don't see an LSP problem with a non-readonly class extending from readonly. As Wikipedia puts it: "Thus, one can define a circle with immutable center and mutable radius as a subtype of an immutable point without violating the history constraint."

Any additional fields added by a child class should generally be invisible to code that depends only on the parent class.

[kocsismate]

Closing it because https://wiki.php.net/rfc/readonly_amendments#proposal_1non-readonly_classes_can_extend_readonly_classes failed.