生成されたバイナリを見ると、ちゃんとprotocolに必要なメソッドが生成されてるぽい。
$ nm /Users/norio/Library/Developer/Xcode/DerivedData/ObjectEncoder-cvflgtjlegddrlbosxxjagoaqewr/Build/Products/Debug/ObjectEncoder.framework/ObjectEncoder|xcrun swift-demangle … 00000000000305e0 t _protocol witness for Swift.SingleValueDecodingContainer.decode(Swift.String.Type) throws -> Swift.String in conformance ObjectEncoder.ObjectDecoder.Decoder : Swift.SingleValueDecodingContainer in ObjectEncoder 000000000002f000 t _protocol witness for Swift.SingleValueDecodingContainer.decode(Swift.Bool.Type) throws -> Swift.Bool in conformance ObjectEncoder.ObjectDecoder.Decoder : Swift.SingleValueDecodingContainer in ObjectEncoder 0000000000030430 t _protocol witness for Swift.SingleValueDecodingContainer.decode(Swift.Double.Type) throws -> Swift.Double in conformance ObjectEncoder.ObjectDecoder.Decoder : Swift.SingleValueDecodingContainer in ObjectEncoder 0000000000030280 t _protocol witness for Swift.SingleValueDecodingContainer.decode(Swift.Float.Type) throws -> Swift.Float in conformance ObjectEncoder.ObjectDecoder.Decoder : Swift.SingleValueDecodingContainer in ObjectEncoder …