Re: [RFC] [DISCUSSION] Immutable/final/readonly properties

As you might noticed, I've not opened the vote yet. Partly because I was improving my implementation as well as the RFC itself (added some words about the inheritance implications), but the main reason is that a question arise in the meanwhile. Namely, "write-once" properties could in principle support covariance. That is, a subclass would be allowed to tighten the property type that is inherited from the parent class. It would be a slight change compared to regular properties that are invariant. All this would be possible because of the quasi-immutable nature of "write-once" properties: they are generally expected to be assigned to only once, in the constructor - which is exempt from LSP checks. There is a gotcha though... In practice, "write-once" properties could be written from places other than the constructor. Although there might not be many practical use-cases for it, the infamous setter injection is certainly one (as shown at https://3v4l.org/DQ3To), in which case property covariance would be a problem. That's why I'm curious about some additional input on the matter. Do you think covariance of "write-once" properties is worth to have even though there might be some edge-cases when it can't be supported perfectly? I'll include this topic a bit later in the RFC as well. In the worst case it could be added to the "Future Scope" section because - and correct me if I'm wrong - we can also add support for it later since it would be a non-breaking change. Cheers, Máté