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

[Zhao Xin]

​It will be better to use `fatalError()` in `default` part that you think
never be needed.​

default:
fatalError("This should never happen!")

Zhaoxin

On Tue, Nov 29, 2016 at 11:25 AM, Dave Abrahams via swift-dev <
swift-dev at swift.org> wrote:

>
> on Sun Nov 27 2016, Slava Pestov <swift-dev-AT-swift.org> wrote:
>
> >> On Nov 27, 2016, at 4:32 PM, David Sweeris via swift-dev <
> swift-dev at swift.org> wrote:
> >>
> >>>
> >>> On Nov 26, 2016, at 5:25 PM, Robert Widmann via swift-dev <
> swift-dev at swift.org
> > <mailto: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.
> >
> > The warning is already gone (and the default cause is required) when
> > the standard library and overlays are built with resilience
> > enabled. The issue is that resilience is off by default.
> >
> > I agree the warning is annoying; we should figure out something.
>
> There should also be a way to write code that will compile without
> warnings whether or not you have resilience enabled. ;-)
>
> --
> -Dave
>
> _______________________________________________
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>
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