Can't call generic function from a generic struct

Question

I have a problem when trying to create a function in extension which would call another generic function. I created very simple example to display the problem :

protocol Target {}
enum api {
    case user
}
extension api: Target {}
protocol Mappable {}

struct User {}
extension User: Mappable {}

struct Provider<A: Target> {

}
extension Provider {

    func requestObject<T: Mappable>(_ token: A, type: T.Type){
        //some code
    }

    func getUser() {

        requestObject(api.user, type: User.self)
        //This code creates Error:  Cannot invoke 'requestObject' with an argument list of type '(api, type: User.Type)'
    }

}

Can anyone help and explain how can create a function getUser() and call requestObject. I know that one solution would be to make struct Provider non-generic, but than It will limit Provider, therefore its not really a solution.

Thanks for help

Answer 1

The issue is that by extending the generic Provider type without any type constraints and declaring the getUser function as you did, the compiler can only infer the type of A to be Api, which is more restrictive than A:Target. You need to use conditional conformance to let the compiler know that the specific getUser implementation should only exist for Provider types specialised with the api type.

extension Provider where A == Api {
    func getUser() {
        requestObject(Api.user, type: User.self)
    }
}

You should also conform to the Swift naming convention, which is upperCamelCase for types, so change api to Api.