Hi,
I copy json.el from github(https://github.com/emacs-mirror/emacs/tree/master/lisp/json.el), override local json.el and compile it.
After I restart emacs, and emacs shows this error in startup:
---
Warning (initialization): An error occurred while loading `c:/Users/caijunqi/AppData/Roaming/.emacs.d/init.el':

error: Attempt to modify read-only object, (error)

To ensure normal operation, you should investigate and remove the
cause of the error in your initialization file. Start Emacs with
the `--debug-init' option to view a complete error backtrace.
---

At 2015-10-04 05:58:11, "Simen Heggestøyl" <simenheg@gmail.com> wrote:
On Sat, Oct 3, 2015 at 11:01 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:

Looks good, please install, Stefan


Done. Thanks for reviewing!


-- Simen