latexdiff with \cite commands gives output with apparently mismatched braces

Question

I have run into a problem with the markup that latexdiff adds around \cite commands. The MWE below, which is supposed to have been generated by latexdiff, doesn't compile because LaTeX doesn't recognise the closing brace for \DIFdel:

ERROR: Extra }, or forgotten \endgroup.

The culprit is \hspace{0pt}: commenting this out prevents this error.

I have solved my immediate problem by manually deleting the string \hspace{0pt} from the difference file wherever it occurs, but obviously I'd rather not do this every time.

I have two questions: (1) why does LaTeX find a problem with the braces here in the first place, since they look matched to my human eyes (and to emacs!)? And (2) is there a way to prevent latexdiff from generating code that fails to compile in this way?

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MWE for LaTeX (note that I've deleted most of the latexdiff preamble)

\documentclass{article}

\usepackage{hyperref}

% 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{\DIFdeltex}[1]{{\protect\color{red}\sout{#1}}}                      %DIF PREAMBLE
%DIF SAFE PREAMBLE %DIF PREAMBLE
\providecommand{\DIFdelbegin}{} %DIF PREAMBLE
\providecommand{\DIFdelend}{} %DIF PREAMBLE
%DIF HYPERREF PREAMBLE %DIF PREAMBLE
\providecommand{\DIFdel}[1]{\texorpdfstring{\DIFdeltex{#1}}{}} %DIF PREAMBLE
%DIF END PREAMBLE EXTENSION ADDED BY LATEXDIFF

\begin{document}
\DIFdelbegin \DIFdel{Some text\mbox{%DIFAUXCMD
\cite{reference}}\hspace{0pt}%DIFAUXCMD
, and some more text.}
\DIFdelend

\bibliography{my_bib}
\end{document}

MWE for latexdiff

old.tex

\documentclass{article}

\usepackage{hyperref}

\begin{document}
Some text \cite{reference}, and some more text.

\bibliography{my_bib}
\end{document}

new.tex

\documentclass{article}

\usepackage{hyperref}

\begin{document}
\bibliography{my_bib}
\end{document}

latexdiff old.tex new.tex > diff.tex

Answer 1

This year (2020) LaTeX underwent some internal changes that introduce incompatibilities with some packages. It seems that ulem.sty is one of them, with the current version ulem 2019/11/18 pre-dating the changes in the LaTeX format.

The differences file produced by latexdiff loads ulem.sty and uses the command \sout (strike out), which leads to the error.

\RequirePackage[normalem]{ulem} %DIF PREAMBLE
...
\providecommand{\DIFdeltex}[1]{{\protect\color{red}\sout{#1}}} %DIF PREAMBLE

As a workaround, until the incomaptibility gets fixed, run latexdiff with the option -t CFONT, which avoids the use of \sout (proposed by @Ant in the comments).

Alternatively, add

\RequirePackage[2019/11/18]{latexrelease}

as the first line in the differences file. This will set back the LaTeX format to a state that is compatible.

Answer 2

Germot already provided a good answer. However, it was not entirely satisfactory. The -t CFONT option changes the formatting (it only uses colour, removing strike-out and underlining). And while setting the latexrelease version to 2019/11/18 worked fine with OP's MWE, it failed in a more complex document.

So I sought to get rid of the real culprit, the \hspace{0pt}.

Since v1.0.1, latexdiff encloses citation (and a few other things such as siunitx commands) in \mbox commands in order to circumvent incompatibilities with ulem styling (the \sout and \uwave used by default). However, this implies that line breaks no longer work within citations. This was fixed in v1.2.1 by adding the \hspace{0pt} after the \mbox. The changelog states that “change should not cause adverse behaviour otherwise”. Well, now it does!

My temporary fix was to edit the latexdiff source (texmf-dist/scripts/latexdiff/latexdiff.pl), and replace the \hspace{0pt} with a \hskip0pt. The resulting PDF is identical to the one obtained with older versions.

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tl; dr:

Find script latexdiff.pl in your texlive installation directory¹, or executable latexdiff in your binaries directory and replace line 2734²:

push(@$retval,"\\mbox{$AUXCMD\n\\" . $1 . $2 . $3 ."}\\hspace{0pt}$AUXCMD\n" );

by

push(@$retval,"\\mbox{$AUXCMD\n\\" . $1 . $2 . $3 ."}\\hskip0pt$AUXCMD\n" );

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¹ E.g. /usr/share/texmf-dist/scripts/latexdiff/latexdiff.pl.
² Assuming you have latexdiff v1.3.1.1.