[swift-dev] [Pitch] Remove "Default will never be executed" Warning?

[David Sweeris]

> On Nov 26, 2016, at 5:25 PM, Robert Widmann via swift-dev <swift-dev at swift.org> wrote:
> 
> Hello all,
> 
> I’ve seen and been a part of a number of conversations recently where talk of planning for “resilient enums”, or even just authors that ship frameworks that will eventually offer a binary option that export enum-based APIs.  The consensus seems to be that the only safe way to deal with this situation is to always cover a `switch` statement with a default, regardless of whether the totality checker thinks you’ve covered all cases.  Currently, the compiler warns when this is the case, as in stdlib
> 
> extension ProcessTerminationStatus {
>   var isSwiftTrap: Bool {
>     switch self {
>     case .exit(_):
>       return false
>     case .signal(let signal):
>       return CInt(signal) == SIGILL || CInt(signal) == SIGTRAP
>     default:
>       // This default case is needed for standard library builds where
>       // resilience is enabled
>       return false
>     }
>   }
> }
> 
> I think this warning doesn’t actually serve a purpose and I’d like to remove it, or at the very least curb it.  Currently, I see three paths forward:
> 
> 1) Do nothing.
> 2) Completely remove the diagnostic
> 3) Only emit the diagnostic when the case-tree is switching over enum(s) declared inside the current module.

Has the “resilient enum” thing been finalized, or is this kind of code just a preventative measure?

Also, if “resilience” is a compiler flag, there's a fourth option: Disable the warning on builds that have “resilience” enabled.

- Dave Sweeris
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