[swift-evolution] stored properties in extensions (was: associated objects)

[Benjamin Spratling]

I’m haven’t read the implementation, but as I see it, “weak” has the effect of storing associated values.  Can that mechanism be made generic?

> +1 from me too for internal extensions; I think this is the logical place to start for now so we can see how many use cases this actually covers. Personally I don't try to add functionality to types from other libraries, and when I have to I try as much as possible to do it via wrapping; obviously this doesn't cover the async type cases and others where it'd be hard to re-wrap the values, but I think I'd like to know more about how common such cases actually are.
> 
> However, allowing stored properties in local extensions is an absolute must for a first step, and would be very useful as a first step.
> 
> 
> The problem I have with doing it for external types is that I just don't see how it can be done efficiently; associated objects means looking up the object to find what its associated values are, which isn't a negligible cost if you're doing it frequently, and it makes me very uncomfortable to think of hiding what is actually happening, as developers may think they're just using a regular property without really appreciating the actual costs involved. At the very least they need to handled through a .associated property or whatever to make it much clearer that these aren't native properties in the normal sense.
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