decode(_:from:) not working when using Int

[vdkdamian]

Issue

I have a function with the following Swift code:

let urlRequest = try self.urlRequest(.get, request: .products, .count, for: .core)
let (data, urlResponse) = try await session.data(for: urlRequest)
        
return try APIManagerDecoder.decode(Int.self, from: data)

This gets transpiled to the following Kotlin code:

val urlRequest = this.urlRequest(ProjectAPI.Method.get, ProjectAPI.Endpoint.products, ProjectAPI.Endpoint.count, for_ = APIManager.TokenOwner.core)
val (data, urlResponse) = session.data(for_ = urlRequest)

return@l APIManagerDecoder.decode(Int::class, from = data)

I'm getting an error from the compiler that refers to the following line:
return@l APIManagerDecoder.decode(Int::class, from = data)

Argument type mismatch: actual type is 'KClass', but 'KClass<uninferred T (of fun decode)>' was expected.

Valid workaround:

I was able to find a workaround as follows:

let urlRequest = try self.urlRequest(.get, request: .products, .count, for: .core)
let (data, urlResponse) = try await session.data(for: urlRequest)
        
return try APIManagerDecoder.decode(IntWrapper.self, from: data).value

Where this is the 'IntWrapper' struct:

struct IntWrapper: Decodable {
    
    var value: Int
    
    init(_ value: Int) {
        self.value = value
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        self.value = try container.decode(Int.self)
    }
}

[marcprux]

I don't know what your APIManagerDecoder looks like, but I expect you are implementing the decode function something like:

func decode<T>(_ type: T.Type, from data: Data) throws -> T where T: Decodable {
    …
}

The problem here is a limitation of Kotlin that Skip cannot overcome, which is that it isn't possible to retroactively conform a type to a protocol when it exists outside of a module. And so we cannot make Int (or String or any of the other standard library types) conform to Decodable.

I'm afraid there's not much we can do to fix this, so I'm afraid your current workaround is the best we can suggest.

[marcprux]

I've moved this over to skip-foundation from skip-lib, since JSONDecoder is a Foundation feature.