[swift-users] Subclass instance passed to closure expecting parent class “cannot convert value of type to expected argument type”

Eric Conner ericwconner at gmail.com
Wed Sep 28 19:43:42 CDT 2016

Hello all!

I hope it's not frowned upon to repost a question here from Stack Overflow,
but I have not found help there.  In any case, the email gets some nice
formatting out of it :-D.

I am trying to create a subclass with a generic type that can be passed
into a completion block, but am getting the following error:

Cannot convert value of type '(ModelListResponse) -> ()' to expected
argument type '(APIResponse) -> ()'

Parent class:

import ObjectMapper
class APIResponse: Mappable {
    var status: String!
    var message: String?

    required init?(map: Map) {}

    func mapping(map: Map) {
        status <- map["status"]
        message <- map["message"]


import ObjectMapper
class ModelListResponse<T: Mappable>: APIResponse {
    var data: ModelList<T>?

    required init?(map: Map) {
        super.init(map: map)

    override func mapping(map: Map) {
        super.mapping(map: map)
        data <- map["data"]

Then I have a function definition:

 func _GET(path: String, parameters: Parameters, completion:@escaping
(APIResponse) -> ())

And I'm trying to call it like so:

// error is on this lineself._GET(path: "/rest/posts", parameters:
parameters) { (response: ModelListResponse<Post>) in
    // ...}

But here I get the error about how ModelListResponse cannot be converted.
Isn't a ModelListResponse an APIResponse?  I don't understand why this does
not work.

I managed to get around the issue by using a generic. Though I'm not sure
why this works and the above code does not. Here is my working declaration:

func _GET<T: Mappable>(path: String, parameters: Parameters,
completion:@escaping (T) -> ()) { }

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