pdfLaTeX hangs with no errors

Question

PdfLateX hangs on a single-line unnumbered equation,

here is a (possibly) minimal example:

\documentclass[english,pointlessnumbers, abstracton, headsepline, open=any]{scrreprt}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{geometry}
\geometry{verbose,tmargin=3cm,bmargin=3cm,headsep=3cm}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\setlength{\parskip}{\medskipamount}
\setlength{\parindent}{0pt}
\usepackage{babel}
\usepackage{prettyref}
\usepackage{refstyle}
\usepackage{float}
\usepackage{units}
\usepackage{textcomp}
\usepackage{url}
\usepackage{amsmath}
\usepackage{makeidx}
\makeindex
\usepackage{graphicx}
\usepackage{esint}
\PassOptionsToPackage{normalem}{ulem}
\usepackage{ulem}
\usepackage{nomencl}
% the following is useful when we have the old nomencl.sty package
\providecommand{\printnomenclature}{\printglossary}
\providecommand{\makenomenclature}{\makeglossary}
\makenomenclature
\usepackage[unicode=true,
 bookmarks=true,bookmarksnumbered=true,bookmarksopen=true,bookmarksopenlevel=4,
 breaklinks=false,pdfborder={0 0 0},backref=false,colorlinks=false]
 {hyperref}

\makeatletter

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands.

\let\pr@chap=\pr@cha
\AtBeginDocument{\providecommand\figref[1]{\ref{fig:#1}}}
\AtBeginDocument{\providecommand\secref[1]{\ref{sec:#1}}}
\AtBeginDocument{\providecommand\eqref[1]{\ref{eq:#1}}}
%% Because html converters don't know tabularnewline
\providecommand{\tabularnewline}{\\}
\RS@ifundefined{subref}
  {\def\RSsubtxt{section~}\newref{sub}{name = \RSsubtxt}}
  {}
\RS@ifundefined{thmref}
  {\def\RSthmtxt{theorem~}\newref{thm}{name = \RSthmtxt}}
  {}
\RS@ifundefined{lemref}
  {\def\RSlemtxt{lemma~}\newref{lem}{name = \RSlemtxt}}
  {}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
% Special symbols as (c), �
\usepackage{textcomp}

% Custom enumeration lists
\usepackage{enumerate}

% Line spacing
% \usepackage{setspace}
% \usepackage[onehalfspacing]{setspace}
\usepackage[doublespacing]{setspace}

\usepackage{        %a4wide,
            ellipsis, fixltx2e, mparhack,       % correct errors with margins
            booktabs, longtable                 % better tables
}  

\usepackage[automark]{scrpage2}     % Page number at bottom right
\automark[chapter]{chapter}
\clearscrheadfoot
\ohead{\\\headmark}     % Paragraph heading
\ofoot[\pagemark]{\pagemark}
\setheadsepline{1pt}    % Header bottom line
\setfootsepline{0pt}    % Footnote top line
\setfootbotline{0pt}    % Footnote bottom line

% Better justify
\usepackage{microtype}

\usepackage{ifpdf}  % part of the hyperref bundle
\ifpdf      % if pdflatex is used

% Set fonts for nicer pdf view
 \IfFileExists{lmodern.sty}{\usepackage{lmodern}}
  {\usepackage[scaled=0.92]{helvet}
    \usepackage{mathptmx}
    \usepackage{courier} }
\fi

% Increase distance between the caption table and the table 
\newcommand{\@ldtable}{}
\let\@ldtable\table
\renewcommand{\table}{ %
                 \setlength{\@tempdima}{\abovecaptionskip} %
                 \setlength{\abovecaptionskip}{\belowcaptionskip} %
                 \setlength{\belowcaptionskip}{\@tempdima} %
                 \@ldtable}

% Table column height
\renewcommand{\arraystretch}{1.5}


\begin{document}

\tracingall

\[
m=m_{0}+\delta_{M}M_{s},                    % INFINITE LOOP
\]

\end{document}

Gives:

\[  ->\x@protect \[\protect \[  

\x@protect #1->\ifx \protect \@typeset@protect \else \@x@protect #1\fi 
#1<-\[
{\ifx}
{true}
{\else}
{\relax}

\[  ->\x@protect \[\protect \[  

\x@protect #1->\ifx \protect \@typeset@protect \else \@x@protect #1\fi 
#1<-\[
{\ifx}
{true}
{\else}
{\relax} ...

Answer 1

A minimal example would have been:

\documentclass{article}
\usepackage{amsmath}
\usepackage{ulem}
\usepackage{fixltx2e}

\begin{document}
\[\]
\end{document}

The tracing shows that \[ is broken. If LaTeX makes a command robust (\DeclareRobustCommand), it uses two definitions for the macro, the original macro and a macro, whose name is appended by a space that contains the real definition of the macro, e.g.:

\DeclareRobustCommand{\.}{bar}
\typeout{[\string\.->\meaning\.]}
\typeout{[\string\.\space->\expandafter\meaning\csname . \endcsname]}

prints the following on the console/.log file:

[\.->macro:->\x@protect \.\protect \.  ]
[\. ->\long macro:->bar]

That gives a little background for analyzing the minimal example:

\documentclass{article}
\newcommand*{\debug}[1]{%
  \typeout{[#1: \string\[->\meaning\[]}%
  \typeout{[#1: \string\[ ->\expandafter\meaning\csname [ \endcsname]}%
}
\debug{initial}
\usepackage{amsmath}
\debug{amsmath}
\usepackage{ulem}
\debug{ulem}
\usepackage{fixltx2e}
\debug{fixltx2e}

\begin{document}
\end{document}

Discussion of the output of \debug:

Initially \[ is not robust:

[initial: \[->macro:->\relax \ifmmode \@badmath \else \ifvmode \nointerlineskip \makebox [.6\linewidth ]{}\fi $$\fi ]

Package amsmath changes the implementation of \[ to use environment equation*. It also defines it using \DeclareRobustCommand:

[amsmath: \[->macro:->\x@protect \[\protect \[  ]
[amsmath: \[ ->\long macro:->\begin {equation*}]

Package ulem does not touch \[, but it defines \MakeRobust:

\def\MakeRobust#1{\expandafter\let
  \csname \expandafter\@gobble\string#1 \endcsname= #1%
 \edef#1{\noexpand\protect \expandafter\noexpand
  \csname\expandafter\@gobble\string#1 \endcsname}
}

It adds the protection layer of \DeclareRobustCommand to add robustness to a fragile macro.

Package fixltx2e also defines \MakeRobust:

\providecommand*\MakeRobust[1]{%
  \@ifundefined{\expandafter\@gobble\string#1}{%
    \@latex@error{The control sequence `\string#1' is undefined!%
      \MessageBreak There is nothing here to make robust}%
    \@eha
  }%
  {%
    \@ifundefined{\expandafter\@gobble\string#1\space}%
    {%
      \expandafter\let\csname
      \expandafter\@gobble\string#1\space\endcsname=#1%
      \edef\reserved@a{\string#1}%
      \def\reserved@b{#1}%
      \edef\reserved@b{\expandafter\strip@prefix\meaning\reserved@b}%
      \edef#1{%
        \ifx\reserved@a\reserved@b
          \noexpand\x@protect\noexpand#1%
        \fi
        \noexpand\protect\expandafter\noexpand
        \csname\expandafter\@gobble\string#1\space\endcsname}%
    }%
    {\@latex@info{The control sequence `\string#1' is already robust}}%
   }%
}

And makes \[ robust:

\MakeRobust\[

Package fixltx2e uses \providecommand, but \MakeRobust is already defined. But the simpler version of package ulem has a serious shortcoming, it does not check, if the command is already made robust. Then following happens. The top macro calls the macro with space and the macro with space contains the original definition. Doing it twice creates a loop, if the macro is called.

[fixltx2e: \[->macro:->\protect \[  ]
[fixltx2e: \[ ->macro:->\x@protect \[\protect \[  ]

The \[␣ calls itself instead of the original macro.

Workaround:

\usepackage{ulem}
\let\MakeRobust\relax % undefine this insufficient version
\usepackage{fixltx2e}

Then the more elaborate version of \MakeRobust of package fixltx2e is used that checks, whether the command is already robust. That avoids the redefinition with the loop.