表达式类型在没有更多上下文的情况下是模棱两可的，快速4.0

 private func handleBase64(message: String) {
            // binary in base64 string
            let noPrefix = message[message.index(message.startIndex, offsetBy: 2)..<message.endIndex]

            if let data = Data(base64Encoded: noPrefix, options: .ignoreUnknownCharacters) {
                client?.parseEngineBinaryData(data)
            }
        }

if let data: Data = Data(base64Encoded: String(noPrefix), options: .ignoreUnknownCharacters) {
    ...
}