Warnings for ambiguous none cases

Paul Hudson      @twostraws

Swift’s optionals are implemented as an enum of two cases: some and none. This gave rise to the possibility of confusion if we created our own enums that had a none case, then wrapped that inside an optional.

For example:

enum BorderStyle {
    case none
    case solid(thickness: Int)
}

Used as a non-optional this was always clear:

let border1: BorderStyle = .none
print(border1)

That will print “none”. But if we used an optional for that enum – if we didn’t know what border style to use – then we’d hit problems:

let border2: BorderStyle? = .none
print(border2)

That prints “nil”, because Swift assumes .none means the optional is empty, rather than an optional with the value BorderStyle.none.

In Swift 5.1 this confusion now prints a warning: “Assuming you mean 'Optional.none'; did you mean 'BorderStyle.none' instead?” This avoids the source compatibility breakage of an error, but at least informs developers that their code might not quite mean what they thought.

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Other changes in Swift 5.1…

• Improvements to synthesized memberwise initializers
• Implicit returns from single-expression functions
• Universal Self
• Opaque return types
• Static and class subscripts
• Matching optional enums against non-optionals
• Ordered collection diffing
• Creating uninitialized arrays

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