> On Jan 4, 2018, at 10:31 PM, Xiaodi Wu via swift-evolution 
> <swift-evolution@swift.org> wrote:
> On Fri, Jan 5, 2018 at 00:21 Cheyo Jimenez <ch...@masters3d.com 
> <mailto:ch...@masters3d.com>> wrote:
> On Jan 4, 2018, at 4:37 PM, Xiaodi Wu <xiaodi...@gmail.com 
> <mailto:xiaodi...@gmail.com>> wrote:
>> On Thu, Jan 4, 2018 at 19:29 Cheyo J. Jimenez <ch...@masters3d.com 
>> <mailto:ch...@masters3d.com>> wrote:
>> We seem to agree that, by virtue of not supporting use in a pattern and 
>> being placed at the end, the feature is a flavor of default. I’m still not 
>> sure I understand why you believe it should not be a flavor of default going 
>> forward.
>>> You still haven’t answered my question, though—what’s the use case for the 
>>> feature you propose?
>> My use case would be distinguishing between compile time known cases vs 
>> “future only” cases (or unknown cases).
>> I understand that the feature you propose would allow you to make such a 
>> distinction, but again, what is your use case for doing so?
> Breaking out early by checking unknown cases first. I admit this is not deal 
> breaker, just a different style I’d like to see supported in the future. 
> I'm still not sure I understand. How can the machine know that it's dealing 
> with an unknown case without first checking if it matches any known case?

I had the same thought as Cheyo.  It isn’t a deal breaker… I like the 
compromise, but I would prefer it trigger only on an actual unknown case (as 
opposed to acting like default). I like to break failure cases out at the top 
when possible. I don’t see any good reason not to support that style.

To answer your question, in the naive sense, it basically is the same question 
as asking if it is a known case (and then taking the inverse). That doesn’t 
mean actually checking each case separately though. For example, if the enum 
cases are internally represented as an unsigned integer, and they are all 
together in a block, the compiler could simply check that it is greater than 
the max known value. You could probably even do a bit mask comparison in some 

If it is inefficient for some reason, the compiler should be free to rearrange 
the order of things, as long as it doesn’t change the outcome.

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