Loading hyperref with \AddToHook{begindocument}

[DrDaum]

When loading the latest version of hyperref with \AddToHook{begindocument} "Missing \endcsname inserted" comes up when using \ref twice:

\documentclass{article}

\AddToHook{begindocument}{ 
	\RequirePackage{hyperref}
}

\begin{document}
Table~\ref{tab1}\\
Table~\ref{tab2}


Caption\label{tab1}\\
Caption\label{tab2}

\end{document}

[muzimuzhi]

Caused by 32a9c4d#diff-d5dc5003cc3fc59b9302ead2d25eb3fe8bc437ce5882facaffe0830cb8c5222cR674 which postpones redefinition of \ref to \AtBeginDocument in nameref.sty.

When hyperref is loaded with \AtBeginDocument/\AddToHook{begindocument}, \AtBeginDocument is the same as \@firstofone hence redefinition of \ref is executed immediately, not postponed. In short, when hyperref is loaded with \AtBeginDocument, you get an illy-defined \ref thus the error.

In general it's a bad idea, hence dangerous, to load any package with \AtBeginDocument. Whether such use case works or not totally depends on the implementation of package and I doubt if it's ever officially supported.

[scottkosty]

@muzimuzhi thank you for your explanation. I think I might have found a similar issue with EuropeCV. I reported it there.

[davidcarlisle]

@scottkosty It looks like europecv could be modified to use begindocument/before hook so not quite as late as begindocument also removing the obsolete UTF-8 handling

$ diff europecv.cls~ europecv.cls
$ diff europecv.cls~ europecv.cls
421,422c421,423
< \RequirePackage{ucs}
< \RequirePackage[utf8x]{inputenc}
---
> %\RequirePackage{ucs}
> %\RequirePackage[utf8x]{inputenc}
> \renewcommand\ecv@utf[1]{{#1}}
449c450
< \AtBeginDocument{%
---
> \AddToHook{begindocument/before}{%

[scottkosty]

@davidcarlisle Thank you! I confirm that fixes things from what I can see. Before I make a pull request, I want to make sure I understand one thing and possibly put it in the release notes. If a user had the line \usepackage[utf8x]{inputenc} in their preamble, their document that used to compile will now fail. But it is in some sense a "good failure" because the user should update their code to avoid the not-recommended utf8x anyway. Is that a reasonable interpretation?

Can you copy/paste your comment above into an answer to
https://tex.stackexchange.com/questions/646747/europecv-fails-with-latex2e-pre-release-because-of-loading-hyperref-in-atbegind ?

Thanks!

[davidcarlisle]

@scottkosty as I think @u-fischer commented elsewhere we may be able to arrange that utf8x plays more nicely with hyperref, but it is very rarely needed these days so a class file like europecv shouldn't load it, then if an individual document does need that they can use the patch described in latex3/latex2e#833 if hyperref has not yet incorporated a similar patch.

[scottkosty]

@davidcarlisle That all sounds good. Thanks for your patient explanations!

[u-fischer]

I close this as resolved. As @muzimuzhi wrote it is a bad idea to load packages in the begindocument. Various packages initialize commands in this hook (including hyperref itself) and timing can get quite tricky if a package is loaded so late.
The better hook here is begindocument/before.