How can \ifx\\\\ be false?

Question

It appears that I have a case where \ifx\\#2\\ is false, even though #2 is empty.

Here is the relevant excerpt from the log file, with \tracingcommands and \tracingmacros turned on:

\lastpageref ->\lastpagereftxt 

\lastpagereftxt #1->\pagesLTS@@pageref #1*\END 
#1<-pagesLTS.arabic

\pagesLTS@@pageref #1*#2\END ->\ifx \\#2\\\pagesLTS@@@pageref {#1}\else \expand
after \pagesLTS@@@pagerefstar \fi 
#1<-pagesLTS.arabic
#2<-
{restricted horizontal mode: \ifx}
{false}

As a result, \pagaesLTS@@@pagerefstar is called, and mayhem results. Not least because this happens in the output routine, while typesetting a complicated page header, and the next token happens to be \cr. But that is all beside the point, which is that this can't happen! And yet it does.

My question: How to debug something like this? Assuming it is not a bug in the TeX engine itself, there must be some natural explanation for it. I have verified (using \show) that \ifx really means \ifx at this point, and I assure you that there is no \unless lurking in the code either. And as far as I know, that exhausts the range of possible explanations. Can you come up with more possibilities I can look for?

Edit: I managed to chop the problem files down to size. Here is my input file:

\errorcontextlines=99
\documentclass{blargh}
\begin{document}
(Dummy text to ensure we get a page.)
\end{document}

And my class file, chopped down from over 300 lines to the bare essentials for this problem, and named blarhg.cls:

\NeedsTeXFormat{LaTeX2e}
\LoadClass{report}
\RequirePackage[pagecontinue=false]{pageslts}

\AtBeginDocument{\thispagestyle{Eks}}

\def\PageText#1#2{Page #1 of #2}
\newtoks\firstpageupperright
\firstpageupperright{\PageText{\thepage}{\lastpageref{pagesLTS.arabic}}}

% Taken from pageslts.sty, so I could add \showtokens
\def\pagesLTS@@pageref#1*#2\END{%
  \ifx\\#2\\% no star
    \pagesLTS@@@pageref{#1}%
  \else% star
    \showtokens{"#2"}%
    \expandafter\pagesLTS@@@pagerefstar%
  \fi%
}

\def\ps@Eks{\let\@mkboth\@gobbletwo
     \def\@oddhead{\hfill \vtop to 0pt{\let\\\cr
                            \halign{\strut\hfil####\cr
                                    \relax\tracingall
                                    \the\firstpageupperright\cr
                                    \llap{}\cr}\vss}}
     \def\@oddfoot{}\let\@evenfoot\@oddfoot
     \let\@evenhead\@oddhead}

Answer 1

Update, in your MWE at the end of the class file change

\let\\\cr

to

\def\\{\cr}

If \\ is \cr then \ifx\\\\ is interesting :-)

---

The following plain TeX shows \ifx\\\\ expanding to false

{\tracingall
\def\\{\hfill\break}
\def\tablecr{{\global\let\\\relax}}
\halign{%
\expandafter
#\cr
\noexpand\ifx\\\\ A \else B \fi
C\cr
}
}

\bye

---

{\ifx: (level 1) entered on line 7}
{false}
{\else: \ifx (level 1) entered on line 7}
{the letter B}