nested if tokens in newenviron

[davidcarlisle]

\Collect@@body doesn't guard against unmatched if tokens in the part of the body scanned so far (actually \collect@@body copied from amsmath doesn't either but there doesn't appear to be a user interface to that here, and in amsmath all the environments are \halign contexts so nested \if perhaps less likely.

see

https://tex.stackexchange.com/a/604786/1090

\documentclass{article}
\usepackage{environ}
\makeatletter
\long\def\Collect@@Body#1\end#2{%
  \edef\begin@stack{%
    \Push@Begins#1\begin\end\expandafter\@gobble\begin@stack}%
  \ifx\@empty\begin@stack
    \endgroup
    \@checkend{#2}%
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
   {\Addto@Envbody{#1}}%
   {\Addto@Envbody{#1\end{#2}}}%
  \process@envbody}
\makeatother

\RenewEnviron{figure}{It's gone!}

\usepackage{ifpdf}
\begin{document}
\begin{figure}
\ifpdf
    \begin{tabular}{l}
        This works without RenewEnviron, ifpdf or tabular!
    \end{tabular}
\else\fi
\end{figure}
\end{document}

[FrankMittelbach]

Perhaps this package should be retired/frozen in favor of \NewDocumentEnvironment{foo}{+b}.... Is there any benefit that is not covered by the extended LaTeX declarations these days?

[davidcarlisle]

@FrankMittelbach I wondered about suggesting the same, but it's used a lot and a small fix wouldn't hurt I think. Also the code is copied straight from amsmath so we could consider fixing it there too, although it's harder to get a plausible example of an \if... used in an alignment

[FrankMittelbach]

@davidcarlisle sure David, but I guess you would need to pick it up then :-).

But regardless of this, I think adding a big notice to such packages that they are kept around for legacy documents and should not be used going forward seems sensible to me. We need to get the number of such extensions down and with xparse as part of the kernel there is little reason to use environ imho. As for amsmath, I think the go forward would be to eventually reimplement the envs using xparse interfaces rather then messing with the current internals.