> On Jan 1, 2018, at 10:47 PM, Chris Lattner via swift-evolution 
> <swift-evolution@swift.org> wrote:
>> On Dec 31, 2017, at 12:14 PM, Matthew Johnson via swift-evolution 
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>> I agree that we need a solution to the problem described.  I also agree that 
>> non-exhaustive is most in keeping with the overall design of Swift at module 
>> boundaries.  However, I believe this proposal should be modified before 
>> being accepted
> Thanks for writing this up - you’ve explained a common concern in an 
> interesting way:
>> This is likely to be a relatively rare need mostly encountered by 3rd party 
>> libraries but it will happen.  When it does happen it would be really 
>> unfortunate to be forced to use a `default` clause rather than something 
>> like a `future` clause which will produce an error when compiled against an 
>> SDK where the enum includes cases that are not covered.  I can imagine cases 
>> where this catch-all case would need to do something other than abort the 
>> program so I do not like the `switch!` suggestion that has been discussed.  
>> The programmer should still be responsible for determining the behavior of 
>> unknown cases.
> ..
>> While library authors have a legitimate need to reserve the right to 
>> introduce new cases for some enums this need can be met without taking away 
>> a useful tool for generating static compiler errors when code does not align 
>> with intent (in this case, the intent being to cover all known cases).  
>> Switch statements working with these kinds of enums should be required to 
>> cover unknown cases but should be able to do so while still being statically 
>> checked with regards to known cases.  
> I think that this could be the crux of some major confusion, the root of 
> which is the difference between source packages and binary packages that are 
> updated outside your control (e.g. the OS, or a dynamic library that is 
> updated independently of your app like a 3rd party plugin).  Consider:
> 1) When dealing with independently updated binary packages, your code *has* 
> to implement some behavior for unexpected cases if the enum is 
> non-exhaustive.  It isn’t acceptable to not handle that case, and it isn’t 
> acceptable to abort because then your app will start crashing when a new OS 
> comes out. You have to build some sort of fallback into your app.
> 2) When dealing with a source package that contributes to your app (e.g. 
> through SwiftPM), *YOU* control when you update that package, and therefore 
> it is entirely reasonable to exhaustively handle enums even if that package 
> owner didn’t “intend” for them to be exhaustive.  When *you* chose to update 
> the package, you get the “unhandled case” error, and you have maximal 
> “knowability” about the package’s behavior.
> It seems that your concern stems from the fact that the feature as proposed 
> is aligned around module boundaries, and therefore overly punishes source 
> packages like #2.  I hope you agree that in case #1, that the feature as 
> proposed is the right and only thing we can do: you really do have to handle 
> unknown future cases somehow.
> If I’m getting this right, then maybe there is a variant of the proposal that 
> ties the error/warning behavior to whether or not a module is a source module 
> vs a binary module.  The problem with that right now is that we have no 
> infrastructure in the language to know this…

YES 👏. This is the conclusion I’ve been coming to as well as I’ve been mulling 
it over.

Exhaustiveness is *only* an issue for modules that are updated independently of 
my app (ie, via an OS or other externally-linked library changing). Modules 
that are shipped with my app are unaffected with this.

I’d really love it if the build system could know that “this module is bundled 
with your app and therefore switches on its enums don't need a default case but 
this other module isn’t bundled with the app, and so therefore switches on its 
enums DO needs a default case”.

Personally, I’m falling on the side of “all enums from external modules are 
non-exhaustive”, but I recognize there are some cases where that’s semantically 
incorrect (Optional<T>, for one; NSComparisonResult for another). In those 
cases, I think I’d prefer to have an attribute on the enum that is overly long 
and scary in order to emphasize that, once shipped, this enum must never change 
again. Something like:

@mustNeverEverEverChangeInAFutureRelease public enum Optional<T> {
        case none
        case some(T)

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