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

[Drew Crawford]

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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