[swift-evolution] Warning when omitting default case for imported enums

[Matthew Johnson]

> On Feb 10, 2017, at 10:55 AM, Tino Heth via swift-evolution <swift-evolution at swift.org> wrote:
> 
>>> I'm not sure if I like the concept of having two kinds of enum.
>> 
>> Why not? Bool-like enums would be declared ‘closed’, and would not require a default case (but adding a new case would then break ABI).
> 
> Well, enums are already (relative) complex, and with this addition, there would be six different flavors.
> Imho it would be less bad if we could recycle existing modifiers, but with a hypothetic "closed" access level added as well, I have strong doubts that the feature carries its weight.
> 
>> For better or worse we need the ability to define enums that admit new cases without breaking ABI. Whether or not this is the default for all enums, or enabled with a special attribute can be designed later when we send out evolution proposals for resilience-related features.
> Intuitively, I thought this should not affect ABI… but no matter what instability this is, I guess it could definitely crash an application that is confronted with an unexpected case ;-)
> 
> Wouldn't it be possible to create an implicit default case for every switch-statement?

What do you suggest this implicit default case do?  Just break?  That seems like the only option besides trapping.  I wouldn’t want the compiler doing either of these after the fact.  If we’re going to allow a library to add new cases without making that a breaking change it must communicate that to users *ahead of time* and the compiler must require a default clause in any switch over that type.

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