Improve syntax on static member look up

[Tim.Wang]

With Extending Static Member Lookup in Generic Contexts implemented in 5.5, the static member lookup feature is improved. However, we still have not covered the following case:

@dynamicMemberLookup
protocol ImageLookup {
    static var images: Images { get }
    static subscript(dynamicMember member: KeyPath<Images, String>) -> UIImage? { get }
}

extension ImageLookup {
    static subscript(dynamicMember member: KeyPath<Images, String>) -> UIImage? {
        return UIImage(named: images[keyPath: member])
   }
}

extension UIImage: ImageLookup {
    static var images: Images {
        Images()
    }
}

struct Images {
    let sample = "sample"
}

let image: UIImage = .sample
let image2 = UIImage.sample

The complier complains on line let image: UIImage = .sample but NOT on line let image2 = UIImage.sample as shown in picture below:

I assume that it is because that sample property is from @dynamicMemberLookup. Could we improve the syntax to support this scenario?

[Tim.Wang]

@xwu Any thoughts on this? Thanks.

[davedelong]

@Tim.Wang

let image: UIImage = .sample
let image2 = UIImage.sample

Well, your dynamic member subscript says that you return a UIImage?, not a UIImage, so it's going to have a hard time matching...

And if you change it to let image: UIImage?, then you're saying you want to look up the .sample name on the Optional<UIImage> type, which does not support dynamic member subscripts.

I'm not sure what the right answer is here.

[Tim.Wang]

Thanks @davedelong , the thing is that even if I change the type to UIImage, the complier is still complaining. Please refer to the code below:

@dynamicMemberLookup
protocol ImageLookup {
    static var images: Images { get }
    static subscript(dynamicMember member: KeyPath<Images, String>) -> UIImage { get }
}

extension ImageLookup {
    static subscript(dynamicMember member: KeyPath<Images, String>) -> UIImage {
        return UIImage(named: images[keyPath: member])!
   }
}

extension UIImage: ImageLookup {
    static var images: Images {
        Images()
    }
}

struct Images {
    let sample = "sample"
}

let image: UIImage = .sample
let image2 = UIImage.sample

The error message is below:

[davedelong]

This, IMO, is a better example. I agree this appears unintuitive

[xwu (Xiaodi Wu)]

@Tim.Wang

@xwu Any thoughts on this? Thanks.

Not yet. There are others who have thought about this more than I.