Hmm for this example I would be more inclined to use an output parameter rather than generics

    func Request(req, resp any) error {
        // send request (http, etc)
        b, err := json.Marshal(req)
        if err != nil {
            return err
        }
        log.Printf("sent request %+v - %s", r, b)

        // read response
        if err := json.Unmarshal([]byte(`{"success": false, "error": "invalid login"}`), resp); err != nil {
            return err
        }

        return nil
    }

But I kind of agree it's nicer to use a return value rather than an output parameter. I'm excited to see what other new uses people come up with for generics!