enum ButtonPropertiesCases {
    case backgroundColor  // Swift 5.2: "Error: enums must not contain stored properties."
    case font
}


@dynamicMemberLookup
class ButtonProperties {
    subscript(dynamicMember key: KeyPath<ButtonPropertiesCases.Type, ButtonPropertiesCases>) -> Bool {
        get { return false }
        set(newValue) { /* do nothing for now */ }
    }
}


let buttonProps = ButtonProperties()
let x = buttonProps.backgroundColor