‘self’ used before all stored properties are initialized

In this case I would suggest you to make the property a variable but hiding it (make it seem like a constant) through a computed property:

class Country {
    let name: String

    private var _capitalCity: City!
    var capitalCity: City {
        return _capitalCity
    }

    init(name: String, capitalName: String) {
        self.name = name
        self._capitalCity = City(name: capitalName, country: self)
    }
}