redefining existing $math$ commands confuses latexdiff

[sim590]

When using a \def instruction, latexdiff inserts macros between \def and its argument.

First file:

\documentclass[12pt]{article}

\begin{document}
  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
  no sea takimata sanctus est Lorem ipsum dolor sit amet.
\end{document}

Second file:

\documentclass[12pt]{article}

\begin{document}
  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
  no sea takimata sanctus est Lorem ipsum dolor sit amet.
  \begin{figure}[t]
    \centering
    \def\cap{titi}
    \caption{   . Name}
    \label{fig:name}
  \end{figure}

  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
  no sea takimata sanctus est Lorem ipsum dolor sit amet.
\end{document}

The diff file:

\documentclass[12pt]{article}
%DIF LATEXDIFF DIFFERENCE FILE
%DIF DEL toto.tex   Fri Apr 12 20:26:40 2019
%DIF ADD titi.tex   Fri Apr 12 20:55:37 2019
%DIF PREAMBLE EXTENSION ADDED BY LATEXDIFF
%DIF UNDERLINE PREAMBLE %DIF PREAMBLE
\RequirePackage[normalem]{ulem} %DIF PREAMBLE
\RequirePackage{color}\definecolor{RED}{rgb}{1,0,0}\definecolor{BLUE}{rgb}{0,0,1} %DIF PREAMBLE
\providecommand{\DIFadd}[1]{{\protect\color{blue}\uwave{#1}}} %DIF PREAMBLE
\providecommand{\DIFdel}[1]{{\protect\color{red}\sout{#1}}}                      %DIF PREAMBLE
%DIF SAFE PREAMBLE %DIF PREAMBLE
\providecommand{\DIFaddbegin}{} %DIF PREAMBLE
\providecommand{\DIFaddend}{} %DIF PREAMBLE
\providecommand{\DIFdelbegin}{} %DIF PREAMBLE
\providecommand{\DIFdelend}{} %DIF PREAMBLE
\providecommand{\DIFmodbegin}{} %DIF PREAMBLE
\providecommand{\DIFmodend}{} %DIF PREAMBLE
%DIF FLOATSAFE PREAMBLE %DIF PREAMBLE
\providecommand{\DIFaddFL}[1]{\DIFadd{#1}} %DIF PREAMBLE
\providecommand{\DIFdelFL}[1]{\DIFdel{#1}} %DIF PREAMBLE
\providecommand{\DIFaddbeginFL}{} %DIF PREAMBLE
\providecommand{\DIFaddendFL}{} %DIF PREAMBLE
\providecommand{\DIFdelbeginFL}{} %DIF PREAMBLE
\providecommand{\DIFdelendFL}{} %DIF PREAMBLE
%DIF LISTINGS PREAMBLE %DIF PREAMBLE
\RequirePackage{listings} %DIF PREAMBLE
\RequirePackage{color} %DIF PREAMBLE
\lstdefinelanguage{DIFcode}{ %DIF PREAMBLE
%DIF DIFCODE_UNDERLINE %DIF PREAMBLE
  moredelim=[il][\color{red}\sout]{\%DIF\ <\ }, %DIF PREAMBLE
  moredelim=[il][\color{blue}\uwave]{\%DIF\ >\ } %DIF PREAMBLE
} %DIF PREAMBLE
\lstdefinestyle{DIFverbatimstyle}{ %DIF PREAMBLE
    language=DIFcode, %DIF PREAMBLE
    basicstyle=\ttfamily, %DIF PREAMBLE
    columns=fullflexible, %DIF PREAMBLE
    keepspaces=true %DIF PREAMBLE
} %DIF PREAMBLE
\lstnewenvironment{DIFverbatim}{\lstset{style=DIFverbatimstyle}}{} %DIF PREAMBLE
\lstnewenvironment{DIFverbatim*}{\lstset{style=DIFverbatimstyle,showspaces=true}}{} %DIF PREAMBLE
%DIF END PREAMBLE EXTENSION ADDED BY LATEXDIFF

\begin{document}
  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
  no sea takimata sanctus est Lorem ipsum dolor sit amet.
  \DIFaddbegin \begin{figure}[t]
    \centering
    \def\DIFaddFL{\cap{titi}
    }\caption{   \DIFaddFL{. Name}}
    \label{fig:name}
  \end{figure}

  \DIFadd{Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
  no sea takimata sanctus est Lorem ipsum dolor sit amet.
 }\DIFaddend\end{document}

The output of (pdflatex):

This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2019/dev/Debian) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./diff.tex
LaTeX2e <2018-12-01>
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2018/09/03 v1.4i Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size12.clo))
(/usr/share/texlive/texmf-dist/tex/generic/ulem/ulem.sty)
(/usr/share/texlive/texmf-dist/tex/latex/graphics/color.sty
(/usr/share/texlive/texmf-dist/tex/latex/graphics-cfg/color.cfg)
(/usr/share/texlive/texmf-dist/tex/latex/graphics-def/pdftex.def))
(/usr/share/texlive/texmf-dist/tex/latex/listings/listings.sty
(/usr/share/texlive/texmf-dist/tex/latex/graphics/keyval.sty)
(/usr/share/texlive/texmf-dist/tex/latex/listings/lstmisc.sty)
(/usr/share/texlive/texmf-dist/tex/latex/listings/listings.cfg)) (./diff.aux)
(/usr/share/texlive/texmf-dist/tex/context/base/mkii/supp-pdf.mkii
[Loading MPS to PDF converter (version 2006.09.02).]
)
! Missing $ inserted.
<inserted text>
                $
l.51     }\caption{   \DIFaddFL{. Name}}

[sim590]

Same thing happens with newcommand also.

[sim590]

After making some more tests, I have discovered that it's because it doesn't like that I name my command \cap which is already defined. I think that there is still an issue here. At least, I can go around it, but this is a bug.

EDIT: it is about redefining math commands. For example: \cup,\cap,\exists, etc.

[ftilmann]

latexdiff only promises to properly process LaTeX, not TeX. Also, it expects all command arguments to be be surround by braces, even if this is not required by latex itself, e.g. \newcommand{\mycommand}{This is my command} should process correctly (except that -- as you noticed -- the fact that \cap normally has a different definition might cause problems later down the line). However \newcommand\mycommand{This is my command} will not work (I am not sure if it would be acceptable in latex, I have never used this form). These limitations are clearly stated in the manual. I will therefore close the issue for now, if you still have problems with the first form, feel free to reopen.

As a workaround, you can surround the offending part of the text with special environment \DIFnomarkup, i.e.

\centering
\begin{DIFnomarkup}
\def\cap{titi}
\end{DIFnomarkup}
\caption{   . Name}

and define in your preamble the environment DIFnomarkup to do absolutely nothing at all.

\newenvironment{\DIFnomarkup}{}{}

(I did not test this, if this workaround does not work as expected, feel free to start a new issue, or reopen this one).

As to your new title - yes, redefining any standard commands with a totally different meaning has great potential to confuse latexdiff. Dealing with this in a universal way would require including the full internal processing of TeX into consideration, so basically this will never be fixed, as way too complicated.