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

[Josh Parmenter]

On Apr 10, 2017, at 9:53 AM, Kevin Nattinger <swift at nattinger.net<mailto:swift at nattinger.net>> wrote:


On Apr 10, 2017, at 9:18 AM, Josh Parmenter via swift-evolution <swift-evolution at swift.org<mailto: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).

Why? Because of preconditionFailure? That COULD be optimized out as per this documentation:

/// * In `-Ounchecked` builds, the optimizer may assume that this function is
///   never called. Failure to satisfy that assumption is a serious
///   programming error.

So I’m not sure if that is the case or not.

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

File a bug, IMO.

Yes - it should probably be looked at by someone who would know if this a bug or the expected behavior.

Best,
Josh


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>> 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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