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.

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

Download all Swift 5.1 changes as a playground Link to Swift 5.1 changes

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