Only supporting v1.2.3 triplet semantic versioning.

I wouldn't go as far as to call it a "gratuitous incompatibility" since there are no Corinna classes right now which would break because of this. But I agree with you that having decimals would be good: Semantic versioning can be an obstacle in early Corinna adoption. If I want to migrate an existing old-style OO class to Corinna while keeping the API, I would need to change from decimal to semantic - and, worse, users of the class might need to change their code, too.

It's the latter concerns that I consider to be incompatibilities. Corinna isn't coming onto a blank stage. It's part of Perl and has to interoperate with what's out there. Particularly if it ever is going to be able to subclass existing modules (which I don't believe is on the table for the MVP) it needs to support the versioning scheme that those modules use.

No native way to support read/write accessors.

I'm not sure what you're expecting here?

Yeah, I let myself confound the issue of immutability with read/write accessors using the same name. To be fair, the overview (which is what I had read most recently) takes the opportunity to take shots at both at about the same time, too.

But now I think I didn't read the RFC carefully enough, because it appears to be at odds with the Overview on the subject of :reader/:writer on attributes. The Overview says under Helper Attributes:

The writer creates a method called set_$name to avoid overloading the meaning of the method name, and will return the invocant. Setting :reader($name) and :writer($name) to the same $name would be an error.

Whereas the RFC section 6.2.3.3 says:

If you explicity (sic) set the name of the writer to the name of the slot, there will be a special case to allow ->method for reading and ->method($new_value) for writing

So it looks like that's been changed? Or am I missing something else?