xr package not working “Incomplete \ifx”

Question

I am trying to use xr for crossreferences between two documents. I've used it before without a problem for a different project.

Here's the error message I get:

(/usr/local/texlive/2018/texmf-dist/tex/latex/tools/xr.sty))
! Incomplete \ifx; all text was ignored after line 12.
<inserted text> 
            \fi 
<*> main

The call to \externaldocument is on line 12 of the main document. I haven't been able to find any problem like this. Any idea what is going on?

Thanks

Edited:

With just these lines in the main.tex it still fails with the same error:

\documentclass{article}
\usepackage{xr}
\externaldocument{supporting}
\begin{document}
Test
\end{document}

The supporting.aux file contains:

\relax 
\providecommand\zref@newlabel[2]{}
\providecommand\hyper@newdestlabel[2]{}
\providecommand\HyperFirstAtBeginDocument{\AtBeginDocument}
\HyperFirstAtBeginDocument{\ifx\hyper@anchor\@undefined
\global\let\oldcontentsline\contentsline
\gdef\contentsline#1#2#3#4{\oldcontentsline{#1}{#2}{#3}}
\global\let\oldnewlabel\newlabel
\gdef\newlabel#1#2{\newlabelxx{#1}#2}
\gdef\newlabelxx#1#2#3#4#5#6{\oldnewlabel{#1}{{#2}{#3}}}
\AtEndDocument{\ifx\hyper@anchor\@undefined
\let\contentsline\oldcontentsline
\let\newlabel\oldnewlabel
\fi}
\fi}
\global\let\hyper@last\relax 
\gdef\HyperFirstAtBeginDocument#1{#1}
\providecommand\HyField@AuxAddToFields[1]{}
\providecommand\HyField@AuxAddToCoFields[2]{}
\bibstyle{pnas-new}
\babel@aux{english}{}
\newlabel{LastPage}{{}{1}{}{page.1}{}}
\xdef\lastpage@lastpage{1}
\xdef\lastpage@lastpageHy{1}
\expandafter\ifx\csname c@section@totc\endcsname\relax\newcounter{section@totc}\fi\setcounter{section@totc}{0}
\expandafter\ifx\csname c@figure@totc\endcsname\relax\newcounter{figure@totc}\fi\setcounter{figure@totc}{0}
\expandafter\ifx\csname c@table@totc\endcsname\relax\newcounter{table@totc}\fi\setcounter{table@totc}{0}
\expandafter\ifx\csname c@NAT@ctr@totc\endcsname\relax\newcounter{NAT@ctr@totc}\fi\setcounter{NAT@ctr@totc}{0}
\expandafter\ifx\csname c@movie@totc\endcsname\relax\newcounter{movie@totc}\fi\setcounter{movie@totc}{0}
\expandafter\ifx\csname c@dataset@totc\endcsname\relax\newcounter{dataset@totc}\fi\setcounter{dataset@totc}{0}
\expandafter\ifx\csname c@SItext@totc\endcsname\relax\newcounter{SItext@totc}\fi\setcounter{SItext@totc}{0}

The external document is a PNAS template from overleaf.

Answer 1

Update xr has been updated with this fix in version

[2019/01/05 v5.04 eXternal References (DPC)]

---

It seems I last edited the code for xr in 1994. It seems like only yesterday...

xr doesn't take kindly to \if tests in the aux file. A fix is relatively simple as shown below, I'll try to get an update into the next release.

\documentclass{article}
\usepackage{xr}
\makeatletter
\long\def\XR@test#1#2#3#4\XR@{%
  \let\XR@next\@gobbletwo
  \ifx#1\newlabel
    \let\XR@next\@firstoftwo%
  \else\ifx#1\@input
     \let\XR@next\@secondoftwo
  \fi\fi
   \XR@next{\newlabel{\XR@prefix#2}{#3}}{\edef\XR@list{\XR@list#2\relax}}%
  \ifeof\@inputcheck\expandafter\XR@aux
  \else\expandafter\XR@read\fi}
\makeatother

\externaldocument{supporting}
\begin{document}
Test
\end{document}

---

Original answer

Hmm actually it is enough to have

\expandafter\ifx\csname wibble\endcsname\relax\relax\fi

in the referenced aux file.

Possibly xr could be more defensive against weird code in the aux file, I'll look later but meanwhile you can process your document if you reference a copy of the auxfile and in the copy delete the block

\expandafter\ifx\csname c@section@totc\endcsname\relax\newcounter{section@totc}\fi\setcounter{section@totc}{0}
\expandafter\ifx\csname c@figure@totc\endcsname\relax\newcounter{figure@totc}\fi\setcounter{figure@totc}{0}
\expandafter\ifx\csname c@table@totc\endcsname\relax\newcounter{table@totc}\fi\setcounter{table@totc}{0}
\expandafter\ifx\csname c@NAT@ctr@totc\endcsname\relax\newcounter{NAT@ctr@totc}\fi\setcounter{NAT@ctr@totc}{0}
\expandafter\ifx\csname c@movie@totc\endcsname\relax\newcounter{movie@totc}\fi\setcounter{movie@totc}{0}
\expandafter\ifx\csname c@dataset@totc\endcsname\relax\newcounter{dataset@totc}\fi\setcounter{dataset@totc}{0}
\expandafter\ifx\csname c@SItext@totc\endcsname\relax\newcounter{SItext@totc}\fi\setcounter{SItext@totc}{0}