Attachment: Download

duplicates:

• SR-2176 Add warning for ambiguous enum value assignment

Issue Description:

Here's a Swift bug that was hiding a real SwiftNIO bug:

struct SelectorEventSet: OptionSet, Equatable {
    typealias RawValue = UInt8

    let rawValue: RawValue

    static let _none = SelectorEventSet(rawValue: 0)
    static let reset = SelectorEventSet(rawValue: 1 << 0)
    static let readEOF = SelectorEventSet(rawValue: 1 << 1)
    static let read = SelectorEventSet(rawValue: 1 << 2)
    static let write = SelectorEventSet(rawValue: 1 << 3)

    init(rawValue: SelectorEventSet.RawValue) {
        self.rawValue = rawValue
    }
}

func test(_ ev: SelectorEventSet) {
    guard ev != .none else { // note how I "forgot" the underscore in front of `._none`
        return
    }
    print("not none")
}

test(._none)

in the line

    guard ev != .none else {

we're comparing ev which is a non-optional❗ SelectorEventSet to .none. I meant to write ._none and the compiler didn't complain at all. .none is the one from Optional.none. So even though ev is non-optional so will never fall into the guard else branch after being lifted to an optional, the Swift compiler lifts ev which is of type SelectorEventSet to Optional<SelectorEventSet>.some(ev) and then compares that to Optional<SelectorEventSet.none (which will never be the same).

Arguably the compiler should not do the optional promotion here because the other side is non-optional and should error. At the very least it should warn that my guard ev != .none doesn't make any sense because it can't hit the else branch.