// the response to find the original request ID.
	mutex   sync.Mutex // protects seq, pending
	seq     uint64
	pending map[uint64]*json.RawMessage
}

// NewServerCodec returns a new [rpc.ServerCodec] using JSON-RPC on conn.
func NewServerCodec(conn io.ReadWriteCloser) rpc.ServerCodec {
	return &serverCodec{
		dec:     json.NewDecoder(conn),
		enc:     json.NewEncoder(conn),
		c:       conn,

	err := client.Call("Arith.Add", args, reply)
	if err == nil {
		t.Error("expected error")
	}
}

func TestServerErrorHasNullResult(t *testing.T) {
	var out strings.Builder
	sc := NewServerCodec(struct {
		io.Reader
		io.Writer
		io.Closer
	}{
		Reader: strings.NewReader(`{"method": "Arith.Add", "id": "123", "params": []}`),
		Writer: &out,
		Closer: io.NopCloser(nil),
	})