Beta6Bug.swift

final class Controller: A {
    var property: String = "" {
        didSet {
            for a in collectionOfA {
                // Cannot assign to property: 'a' is a 'let' constant
                a.property = property
            }
        }
    }

    var collectionOfA: [A] = []
}

// This protocol declaration causes the error to be thrown...
protocol A {
    var property: String { get set }
}

// ...but marking it as a `class` protocol results in no error being raised.
protocol A: class {
    var property: String { get set }
}