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

[Drew Crawford]

On April 11, 2017 at 11:38:05 AM, Joe Groff (jgroff at apple.com) wrote:

By design, Swift avoids making semantic rules based on that kind of analysis, since it would be brittle and difficult to describe when the compiler can and can't see that a condition holds nonlocally like this.
Swift *currently implements* semantic rules based on this kind of analysis.  Exhibit A:

func foo() {

    let a: Bool

    if UUID().uuidString == "non-local condition" {

        a = true

    }

    else {

        preconditionFailure("Don't initialize a")

    }

    print("\(a)") //NOT: error: a is uninitialized

}

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