bug#40693: 28.0.50; json-encode-alist changes alist

[Ivan Andrus]

I recently filed #40692 (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=40692) 
about a problem I was having with magit forge using creating the wrong title 
because an alist created in a function contained a single constant at the end 
and it was being changed every time.  I still find it surprising that a 
back-quoted list would behave that way, but I suspect it will be considered 
desired, or at least expected behavior.

I was able to track it down to the actual function that was causing the change: 
json-encode-alist when json-encoding-object-sort-predicate is set.  I have it 
set, and indeed it's extremely useful to view JSON that in a meaningful order.  
I don't think that anyone would expect that encoding json would change the 
underlying data, so I think json-encode-alist should be changed to make a copy 
to avoid this situation.


  (require 'json)

  (defun fun-withdraw (amount)
    (json-encode-alist
     `((tamount . , amount)
       (const . some-constant))))

  (fun-withdraw 12) ;; Run this a few times and nothing will change

  (setq json-encoding-object-sort-predicate #'string<)

  (fun-withdraw 12) ;; Now it will change every time


It's late now, but if I have some time in the next few days, I may submit a 
patch since it seems like a simple enough change.

-Ivan


In GNU Emacs 28.0.50 (build 18, x86_64-apple-darwin18.7.0, NS appkit-1671.60 
Version 10.14.6 (Build 18G4032))
of 2020-04-10 built on iandrus-macOS
Repository revision: 965390ca5f206c6358014574fe5140ba40850391
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.6

Configured using:
'configure
PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig:/usr/local/opt/libxml2/lib/pkgconfig
--with-sound=yes --with-ns --with-modules --with-file-notification=yes
--enable-gcc-warnings=warn-only --with-xpm --with-jpeg --with-tiff
--with-gif --with-png --with-rsvg --with-xml2 --with-imagemagick
--with-json --with-xft --with-libotf --with-gnutls=no --with-makeinfo
--with-libgmp'

Configured features:
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