↓ Swifty じゃなくないですか？引数の型は
Encoder でなく T: Encoder であるべきでは？ public protocol Encodable { /// Encodes `self` into the given encoder. /// /// If `self` fails to encode anything, `encoder` will encode an empty keyed container in its place. /// /// - parameter encoder: The encoder to write data to. /// - throws: An error if any values are invalid for `encoder`'s format. func encode(to encoder: Encoder) throws }