`\dots` not working correctly with `\cong` about latex2e

[davidcarlisle]

we could pull \DOTSB out of the protection but I think it might be better to use \protected\def and then make the lookahead cope with a \protected meaning (similar to how we added support for \newcommand aka \long\def a while back)

[davidcarlisle]

\documentclass{article}

  % Any preamble code goes here
\usepackage{amsmath}
\makeatletter

{\uccode`9=`\p %
  \uppercase{\gdef\stripprotected@#1#2#3\relax{%
      \ifx9#2 \@xp\@xp\@xp\zap@to@space\fi}}}


\def\mdots@@{\gdef\thedots@{\dotso@}%
 \ifx\@let@token\boldsymbol
   \gdef\thedots@\boldsymbol{\boldsymboldots@}%
 \else
   \ifx,\@let@token \gdef\thedots@{\dotsc}%
   \else
     \ifx\not\@let@token
       \gdef\thedots@{\dotsb@}%
     \else
       \keybin@
       \ifgtest@ % if \keybin@ test
         \gdef\thedots@{\dotsb@}%
       \else
         \xdef\meaning@{\meaning\@let@token. .........}%
         \xdef\meaning@@{\@xp\striplong@\meaning@\relax\meaning@}%
% NEW BIT
         \xdef\meaning@@{\@xp\stripprotected@\meaning@@\relax\meaning@@}%
% END NEW BIT
         \@xp\math@\meaning@\math@
         \ifgtest@ % if \mathxxx test
           \@xp\mathch@\meaning@\mathch@
           \ifgtest@ % if \mathchar
             \@xp\getmathch@\meaning@\getmathch@
           \fi % end if \mathchar
         \else  % \not \mathxxx
             \@xp\Umathch@\meaning@"0"\Umathch@
             \ifgtest@ % if \Umathchar
             \else % else not \Umathcar
           \@xp\macro@\meaning@@\macro@
           \ifgtest@ % if macro test
             \@xp\not@\meaning@\not@
             \ifgtest@ % if macro starts \not test
               \gdef\thedots@{\dotsb@}%
             \else% else not \not
               \@xp\DOTS@\meaning@\DOTS@
               \ifgtest@ % \if DOTS
                 \ifcase\number\DOTSCASE@ %ifcase dots
                   \gdef\thedots@{\dotsb@}%
                 \or\gdef\thedots@{\dotsi}\else
                 \fi % endifcase dots
               \else % not macro starts \DOTS
                 \@xp\math@\meaning@\math@
                 \ifgtest@ % \if macro starts \mathxxxx
                   \@xp\mathbin@\meaning@\mathbin@
                   \ifgtest@ % if macro starts \mathbin
                     \gdef\thedots@{\dotsb@}%
                   \else % not macro starting \mathbin
                     \@xp\mathrel@\meaning@\mathrel@
                     \ifgtest@ % if macro starts \mathrel
                       \gdef\thedots@{\dotsb@}%
                     \fi % endif macro starts \mathrel (no else)
                   \fi % endif macro starts \mathbin
                 \fi % endif macro starts with \mathxxx (no else)
               \fi % endif macro starts \DOTS else
             \fi % end macro  starting \not \ifgtest@ test (no else)
             \else
               \@xp\thecharacter@\meaning@\thecharacter@
             \fi % end macro \ifgtest@ test (no else)
           \fi % end if \Umathchar test
         \fi % end \math@   \ifgtest@
       \fi % end \keybin@ \ifgtest@ test (no else)
     \fi % end if \not (no else)
   \fi % end if comma (no else)
 \fi % end if boldsymbol (no else)
 \thedots@}

\protected
\def
  \cong{\mathrel{\mathpalette\@vereq\sim}} % congruence sign
\makeatother
\begin{document}

  % Demonstration of issue here
$\equiv\dots\equiv$

$\cong\dots\cong$
  
\end{document}

[FrankMittelbach]

we could pull \DOTSB out of the protection but I think it might be better to use \protected\def and then make the lookahead cope with a \protected meaning (similar to how we added support for \newcommand aka \long\def a while back)

agreed, but then we should alter this for all symbols that use \DOTSB etc, shouldn't we?

By the way where is \DOTSB in your code sample?

[davidcarlisle]

agreed, but then we should alter this for all symbols that use \DOTSB etc, shouldn't we?

probably or at least such a change gives a way forward for that to work.

By the way where is \DOTSB in your code sample?

it is not needed as once the lookahead can get past the \protected (or we could make it work with the existing \DeclareRobstCommand \protect version) then it sees the \mathrel so defaults to centred dots just a it does with \equiv in the example (which also has no \DOTS... prefix).