Hi Iijie,
swift extensions cannot have stored properties, they can only have computed properties. even if you are able to set some value to a computed property, it will give you value which you write in get {}. in other word you may not use its value as you do with stored properties. If I am not wrong you are trying to store some value to it.
Still if you want to try `set` thing, here is the modified code for you:

public protocol MyHelperCompatible {
    associatedtype someType
    var my: someType { get set }
}
public extension MyHelperCompatible {
    public var my: MyHelper<Self> {
        get {
            return MyHelper(self)
        }
        set(newValue) {
            my = newValue
        }
    }
}
public struct MyHelper<Base> {
    let base: Base
    init(_ base: Base) {
        self.base = base
    }
}
extension UIView: MyHelperCompatible {}
extension MyHelper where Base: UIView {
    public var width: CGFloat {
        set(newValue) {
            width = newValue
        }
        get {
            return 0.1 //some computed value
        }
    }
}

let me know if this makes sense.
cheers.