https://stackoverflow.com/q/78398747/1271826

FetchData.swift

func fetchData<M: Decodable>(target: T, responseClass: M.Type = M.self, completion: @escaping(Result<M, Error>) -> Void) {
    let method = Alamofire.HTTPMethod(rawValue: target.methods.rawValue)
    let headers = Alamofire.HTTPHeaders(target.headers ?? [:])
    let params = buildParams(task: target.task)
    AF.request(target.baseUrl + target.path, method: method, parameters: params.0, encoding: params.1, headers: headers)
        .validate()
        .responseDecodable(of: M.self) { response in
            switch response.result {
            case .success(let value): completion(.success(value))
            case .failure(let error): completion(.failure(error))
            }
        }
}

Three observations:

1. No need to re-encode and re-decode. Just look at response.result.
2. As a minor refinement, made responseClass an optional parameter (as often the type can be inferred from the context). But I kept the parameter for those cases where the compiler cannot infer the type automatically).
3. I also used validate to check the status code for you.

Also note, in the process of simplifying this, I have retired the use of try?, which throws away any meaningful diagnostic information. Always pass along the meaningful error. If the caller doesn't care about the details, it can disregard the error’s details if it wants. But in those cases where you actually have parsing errors, it is exceptionally useful to have the details (so you know which field(s) caused the problem).