[swift-evolution] switch must be exhaustive, consider adding a default clause

[Kevin Nattinger]

> On Apr 10, 2017, at 9:18 AM, Josh Parmenter via swift-evolution <swift-evolution at swift.org> wrote:
> 
> case .none isn’t handled.

It shouldn’t need to be handled in the second switch because it’s impossible. Not just developer thinks “impossible” but easily provable by static analysis the compiler is doing it anyway—I wouldn’t be surprised if including the case resulted in an “unreachable code” warning (in fact, it probably should).

> This should probably be on the swift-users list though, no?

File a bug, IMO. 

> Best,
> Josh
> 
> 
> 
> On Apr 8, 2017, at 11:29 AM, Drew Crawford via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org><mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>> wrote:
> 
> 
> 
> Is there a good reason we do not compile this:
> 
> import UIKit
> 
> func foo(operation: UINavigationControllerOperation) {
>    switch(operation) {
>    case .push: /* snip */ break
>    case .pop: /* snip */ break
>    default:
>        preconditionFailure("This is a silly operation")
>    }
>    switch(operation) {
>        case .push: /* snip */ break
>        case .pop: /* snip */ break
>         //error: Switch must be exhaustive, consider adding a default clause
>    }
> }
> 
> The switch *is* exhaustive, because the default case is unreachable.  The compiler could infer as much from branch analysis.
> 
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