Protocol retrofitting for constrained generic extension in swift

protocol-retrofit.swift

let myString: String? = nil

protocol PossiblyEmpty {
    var isEmpty: Bool { get }
}

extension String: PossiblyEmpty {}

extension Optional where Wrapped: PossiblyEmpty {
    var isEmpty: Bool {
        switch self {
        case .None: return true
        case .Some(let value): return value.isEmpty
        }
    }
}

myString.isEmpty

@jeremiegirault

Swift 3 update:
Change:

.none
.some