[swift-evolution] [Review] SE 0192 - Non-Exhaustive Enums

[Matthew Johnson]

> On Jan 2, 2018, at 3:41 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Tue, Jan 2, 2018 at 3:27 PM, Kevin Nattinger <swift at nattinger.net <mailto:swift at nattinger.net>> wrote:
> [...]
> 
>>> in what other circumstances do we insist that the compiler inform the end user about future additions to the API at compile time?
>> 
>> This isn’t a request for the compiler to inform the user about future additions to an API.  It is a request to validate the compiler’s knowledge of the current state of an API with the current state of the source code. 
>> 
>> Well, it's of course impossible to inform the user about future additions, so that's poorly phrased on my part. It's about the compiler informing the end user about *new* additions, part of the *current* state of the API, that have cropped up since the user last revised the code when the API was in a *previous* state (or, indistinguishably, members of which a user is unaware regardless of the temporal sequence of when such members were added). In what other circumstances do we insist that the compiler perform this service?
> 
> Enums. That's literally how they work today. You are arguing in favor of actively removing compiler-aided correctness.
> 
> There's also protocol requirements
> 
> No, that's now how enums work today, and it's not how protocol requirements work today. Enums today are all semantically exhaustive; if a case is added in a later version of a library, it's semantically a *different* enum type that happens to share the same name. Not considering all the cases of an exhaustive enum is an _error_, not a _warning_, because there is no basis on which to proceed. This will not change with the proposal. Likewise, adding a protocol requirement without a default implementation is source-breaking. The result is a compiler _error_.
> 
> The question is, what non-source breaking API additions today cause the compiler to inform the end user of such additions?

Posing the question this way takes it as a given that adding a case to a resilient enum is non-source breaking with a full stop.  The position of everyone asking for something like `future` / `unknown` as an alternative to `default` is exactly that this should not be the case.  Instead, adding a case should always be binary compatible and should be source compatible by default, but authors should have the ability to opt-in to making case additions be source-breaking for individual switch statements.  

When you view it this way we are not asking the compiler to inform us of a non-source breaking addition.  We are asking the compiler to treat an addition as source breaking in a specific context.

> The answer is: none whatsoever. Not new methods or properties on a type, not even new protocol requirements that have a default implementation.
> 
> 
> and, arguably, deprecated methods with a proper message ("use foo instead").
> 
> 



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