Xcode更新导致'dataTask(with:completionHandler:)'调用歧义错误？

这段代码数月前可正常运行，推测因Xcode更新导致错误，且三处实现GitHub请求的同类代码均出现该问题，确认未修改相关代码。错误触发于let task = URLSession.shared.dataTask(with: request) { data, response, error in guard语句处。

原代码 #

func sendToGist(payload : GistExport.Output) {
    guard let data = encode(src: payload) else {
        print("no data")
        return
    }
    var urlString = "https://api.github.com/gists"
    if userProject!.gistID != nil {urlString += "/\(userProject!.gistID!)"}
    let url = URL(string: urlString)!
    var request = URLRequest(url: url)
    request.httpBody = data.data(using: String.Encoding.utf8)
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    request.setValue("application/vnd.github+json", forHTTPHeaderField: "Accept")
    request.setValue("Bearer \(StartController.token)", forHTTPHeaderField: "Authorization")
    request.httpMethod = userProject?.gistID == nil ? "POST" : "PATCH"

    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        guard
            let data = data,
            let response = response as? HTTPURLResponse, error == nil
                
        else {                                                               // check for fundamental networking error
            print("error", error ?? URLError(.badServerResponse))
            return
        }
        
        guard (200 ... 299) ~= response.statusCode else {                    // check for http errors
            print("statusCode should be 2xx, but is \(response.statusCode)")
            print("response = \(response)")
            return
        }
        do {
            let decodedData = try JSONDecoder().decode(GistResult.self, from: data)
            print("done \(decodedData)")
            self.userProject!.updateCurrentID(decodedData.id)
        } catch {
            print(error) // parsing error
            if let responseString = String(data: data, encoding: .utf8) {
                print("responseString = \(responseString)")
            } else {
                print("unable to parse response as string")
            }
        }
    }
    task.resume()
}

问题分析与解决方案 #

最可能的原因是闭包参数与内部变量重名：闭包传入的data、response、error参数，和guard语句中重新绑定的data、response变量同名。新版本Swift对变量命名的歧义检查更严格，因此触发编译错误。

修改方案：给闭包参数添加前缀或改名，避免与内部变量冲突：

let task = URLSession.shared.dataTask(with: request) { taskData, taskResponse, taskError in
    guard
        let data = taskData,
        let response = taskResponse as? HTTPURLResponse, taskError == nil
            
    else {
        print("error", taskError ?? URLError(.badServerResponse))
        return
    }
    
    guard (200 ... 299) ~= response.statusCode else {
        print("statusCode should be 2xx, but is \(response.statusCode)")
        print("response = \(response)")
        return
    }
    do {
        let decodedData = try JSONDecoder().decode(GistResult.self, from: data)
        print("done \(decodedData)")
        self.userProject!.updateCurrentID(decodedData.id)
    } catch {
        print(error)
        if let responseString = String(data: data, encoding: .utf8) {
            print("responseString = \(responseString)")
        } else {
            print("unable to parse response as string")
        }
    }
}

其他排查点 #

1. 检查项目Swift版本：Xcode更新后项目的Swift语言版本可能自动升级，可在项目设置中确认是否与之前版本一致，若需兼容可尝试降级Swift版本。
2. 查看具体错误提示：若编译错误提示为“ambiguous use of 'data'”，则可确认是变量重名问题，按上述方案修改即可。
3. 批量处理同类代码：三处同类代码均存在该问题，可统一替换闭包参数命名，快速解决所有报错。

内容的提问来源于stack exchange，提问作者Dan Donaldson