bug#35074: 12.1.2; TeX-complete-symbol doesn't work in math mode for user-defined command

[Andrea Greselin]

Hello,

I can't find a way to make AUCTeX complete user-defined commands in math mode.

As an example, in the document below TeX-complete-symbol works outside
of math mode, but if I prepend a dollar sign to the command name it
doesn't complete it, returning `TeX-complete-symbol: Can’t find
completion for "\rnd".`

-------------------------------------------
\documentclass{article}

\newcommand{\rndvar}{X}

\begin{document}

\rnd%      TeX-complete-symbol works here
$\rnd$%    but not here,
$\rnd%     nor here.

\end{document}
-------------------------------------------

I have posted a question on TeX.SE and I refer to the answer at
https://tex.stackexchange.com/a/482631/35903 for a full (and better
formatted) explanation of the faulty behaviour, which is the same as
in my system. Basically, it is `LaTeX-math-list` not being updated
with the name of the new command that appears to cause the issue.

Thank you for maintaining AUCTeX, it's an awesome tool! :-)

Best wishes,
Andrea

Emacs  : GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.21)
 of 2017-09-22, modified by Debian
Package: 12.1.2

current state:
==============
(setq
 AUCTeX-date "2019-02-08"
 window-system 'x
 LaTeX-version "2e"
 TeX-style-path '("~/.emacs.d/auctex"
                  "/home/andrea/.emacs.d/elpa/auctex-12.1.2/style"
                  "/home/andrea/.emacs.d/auctex/auto"
                  "/home/andrea/.emacs.d/auctex/style" "auto" "style")
 TeX-auto-save nil
 TeX-parse-self nil
 TeX-master t
 TeX-command-list '(("TeX"
                     "%(PDF)%(tex) %(file-line-error) %`%(extraopts) 
%S%(PDFout)%(mode)%' %t"
                     TeX-run-TeX nil
                     (plain-tex-mode ams-tex-mode texinfo-mode) :help
                     "Run plain TeX")
                    ("LaTeX" "%`%l%(mode)%' %T" TeX-run-TeX nil
                     (latex-mode doctex-mode) :help "Run LaTeX")
                    ("Makeinfo" "makeinfo %(extraopts) %t"
                     TeX-run-compile nil (texinfo-mode) :help
                     "Run Makeinfo with Info output")
                    ("Makeinfo HTML" "makeinfo %(extraopts) --html %t"
                     TeX-run-compile nil (texinfo-mode) :help
                     "Run Makeinfo with HTML output")
                    ("AmSTeX"
                     "amstex %(PDFout) %`%(extraopts) %S%(mode)%' %t"
                     TeX-run-TeX nil (ams-tex-mode) :help "Run AMSTeX")
                    ("ConTeXt"
                     "%(cntxcom) --once --texutil %(extraopts) %(execopts)%t"
                     TeX-run-TeX nil (context-mode) :help
                     "Run ConTeXt once")
                    ("ConTeXt Full"
                     "%(cntxcom) %(extraopts) %(execopts)%t"
                     TeX-run-TeX nil (context-mode) :help
                     "Run ConTeXt until completion")
                    ("BibTeX" "bibtex %s" TeX-run-BibTeX nil t :help
                     "Run BibTeX")
                    ("Biber" "biber %s" TeX-run-Biber nil t :help
                     "Run Biber")
                    ("View" "%V" TeX-run-discard-or-function t t :help
                     "Run Viewer")
                    ("Print" "%p" TeX-run-command t t :help
                     "Print the file")
                    ("Queue" "%q" TeX-run-background nil t :help
                     "View the printer queue" :visible
                     TeX-queue-command)
                    ("File" "%(o?)dvips %d -o %f " TeX-run-dvips t t
                     :help "Generate PostScript file")
                    ("Dvips" "%(o?)dvips %d -o %f " TeX-run-dvips nil
                     t :help "Convert DVI file to PostScript")
                    ("Dvipdfmx" "dvipdfmx %d" TeX-run-dvipdfmx nil t
                     :help "Convert DVI file to PDF with dvipdfmx")
                    ("Ps2pdf" "ps2pdf %f" TeX-run-ps2pdf nil t :help
                     "Convert PostScript file to PDF")
                    ("Glossaries" "makeglossaries %s" TeX-run-command
                     nil t :help
                     "Run makeglossaries to create glossary file")
                    ("Index" "makeindex %s" TeX-run-index nil t :help
                     "Run makeindex to create index file")
                    ("upMendex" "upmendex %s" TeX-run-index t t :help
                     "Run upmendex to create index file")
                    ("Xindy" "texindy %s" TeX-run-command nil t :help
                     "Run xindy to create index file")
                    ("Check" "lacheck %s" TeX-run-compile nil
                     (latex-mode) :help
                     "Check LaTeX file for correctness")
                    ("ChkTeX" "chktex -v6 %s" TeX-run-compile nil
                     (latex-mode) :help
                     "Check LaTeX file for common mistakes")
                    ("Spell" "(TeX-ispell-document \"\")"
                     TeX-run-function nil t :help
                     "Spell-check the document")
                    ("Clean" "TeX-clean" TeX-run-function nil t :help
                     "Delete generated intermediate files")
                    ("Clean All" "(TeX-clean t)" TeX-run-function nil
                     t :help
                     "Delete generated intermediate and output files")
                    ("Other" "" TeX-run-command t t :help
                     "Run an arbitrary command")
                    )
 )