Manu:

Okay, so the consensus is, it doesn't currently exist, but there is no real
resistance, and is tentatively planned?
Sounds good to me.

As far as I know there are no concrete ideas yet for the semantics and precise usage of this feature. And I think there are different ideas regarding what this feature has to do (example: I think of it as as a user-defined extension of the type system. Other people think of this feature more like the C#, Scala or Java ones).

So if you want this feature to happen and you have good ideas, then I suggest you to write down a little concrete proposal of definition syntax/semantics (implementation isn't needed now).

Bye,
bearophile