AucTeX mistakes Tikz warning as error

Question

AucTeX 11.90 (Emacs 25.2) gives the following error instead of a warning when using Tikz Externalize:

! Package tikz Warning: The key 'up to date check=md5' is impossible, there is no macro to compute MD5. Falling back to 'up to date check=diff'.

Though my PDF compiles fine, I find the false error quite annoying. Any help in resolving this will be appreciated.

I have the following custom set variables in my init file:

'(TeX-debug-bad-boxes t)
'(TeX-debug-warnings t)
'(TeX-display-help nil)
'(TeX-engine (quote xetex))
'(TeX-error-overview-open-after-TeX-run t)
'(TeX-source-correlate-mode t)
'(TeX-source-correlate-start-server t)

MWE:

\documentclass[12pt]{article}
\usepackage{fontspec,realscripts}
\defaultfontfeatures{Ligatures=TeX,Scale=MatchLowercase} 
\setmainfont[Scale=1]{Libertinus Serif}

\usepackage{tikz}
\usetikzlibrary{external} \tikzexternalize[prefix=tikz/]

\begin{document}
\begin{figure}
  \centering
  \begin{tikzpicture}
    \draw (0,0) -- (1,0) -- (1,1) -- (0,1) -- (0,0);
  \end{tikzpicture}
\end{figure}
\end{document}
%%% Local Variables:
%%% TeX-command-extra-options: "-shell-escape"
%%% End:

Answer 1

Exclamation marks are usually employed to denote errors, rather than warnings. Consider for example this document:

\documentclass{article}
\usepackage{does-not-exist}
\begin{document}
Hello world!
\end{document}

This results in this error in the log:

! LaTeX Error: File `does-not-exist.sty' not found.

Also fatal errors starts with a ! (try compiling with plain TeX a document containing only \doesnotexist: two different errors starting with ! are thrown). See also https://en.wikibooks.org/wiki/LaTeX/Errors_and_Warnings#Error_messages Also the TeXbook indicates that error messages start with the exclamation mark.

Thus, I believe it's an unfortunate decision by TikZ to emit a warnings that starts with an exclamation mark. Also other TeX editors (I tried AUCTeX, TeXstudio and Texworks) have the same issue here. This could be fixed in AUCTeX (but it's very tricky, because there are several different error messages starting with an exclamation mark and there are several places in AUCTeX's code where ! is assumed to start an error message), but emitting warnings compliant with long-standing habits would alleviate the pain of front-end developers.

---

For this particular case, the warning can be avoided by using the option suggested in the message:

\tikzexternalize[prefix=tikz/,up to date check=diff]