[swift-users] Handle deprecated enum cases from a library

[Dennis Weissmann via swift-users]

Hi swift-users,

In a project (iOS 11 only) we use Touch ID for authentication and handle errors 
like so:

private func handleLocalAuthenticationError(_ error: LAError) {
    switch error.code {
    case .userCancel, .appCancel, .systemCancel:
        // Handle cancelation
    case .authenticationFailed:
        // Handle failure
    case .passcodeNotSet:
        // Handle passcode absence
    case .touchIDNotEnrolled:
        // Handle no Touch ID enrollment
    case .touchIDNotAvailable:
        // Handle no Touch ID availabe
    case .touchIDLockout:
        // Handle Touch ID Lockout
    case .userFallback:
        // Handle user fallback
    case .invalidContext:
        // Handle failure with invalid context
    case .notInteractive:
        // Handle no interaction allowed error
    }
}

This worked fine until recently, when with the release of iOS 11 and the 
introduction of Face ID, `.touchIDLockout`, `.touchIDNotEnrolled` and 
`.touchIDNotAvailable` were deprecated in favor of the newly introduced 
`.biometryLockout`, `.biometryNotEnrolled` and `.biometryNotAvailable`.

When we link against the latest SDK we need to add those in order to compile 
(which is obviously good).

Now when we add those cases, the compiler shows the following warnings:

<unknown>:0: warning: 'touchIDLockout' was deprecated in iOS 11.0: use 
LAErrorBiometryLockout
<unknown>:0: warning: 'touchIDNotEnrolled' was deprecated in iOS 11.0: use 
LAErrorBiometryNotEnrolled
<unknown>:0: warning: 'touchIDNotAvailable' was deprecated in iOS 11.0: use 
LAErrorBiometryNotAvailable

The problem here is, that we can't remove the old ones and replace them with 
their new variants because for Swift the enum is then not exhaustive (which 
also makes sense, they're only deprecated, not removed after all).

Even though not optimal (read "really bad because not exhaustive") I thought 
about removing the old cases and adding a default label:

private func handleLocalAuthenticationError(_ error: LAError) {
    switch error.code {
    case .userCancel, .appCancel, .systemCancel:
        // Handle cancelation
    case .authenticationFailed:
        // Handle failure
    case .passcodeNotSet:
        // Handle passcode absence
    case .biometryNotEnrolled:
        // Handle no biometry enrollment
    case .biometryNotAvailable:
        // Handle no biometry availabe
    case .biometryLockout:
        // Handle biometry Lockout
    case .userFallback:
        // Handle user fallback
    case .invalidContext:
        // Handle failure with invalid context
    case .notInteractive:
        // Handle no interaction allowed error
    default:
        // hopefully only deprecated cases
        break
    }
}

However, in this case the compiler warns that "Default will never be executed".

Is there any way of getting rid of these warnings (preferably the first 3 so 
that we don't have to have default clause)?

Thanks!

- Dennis 
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