Extra \endgroup in \AddToHook

[ysalmon]

Brief outline of the bug

The command \AddToHook causes an error in special circumstances.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\test}{m}{
    \group_begin:
    \cs_set_eq:cN { ~ } \space
    \group_end:
}

\AddToHook{cmd/test/before}{\relax}
\ExplSyntaxOff

\begin{document}
nothing
\end{document}

Note the error goes away if the {m} spec is replaced with {}. Also note I have no idea about \cs_set_eq:cN { ~ } \space, this is minimised from a function in a package I did not write (Piton, function \__piton_piton_standard).

Log file (required) and possibly PDF file

texstudio_VuFlvC.log
No PDF file is produced.

[muzimuzhi]

Similar: #1099.

[u-fischer]

unrelated but you should not patch internal commands of other packages.

[FrankMittelbach]

I think this ends up being an unfortunate restriction. It is documented that it is not possible to use generic command hooks with every command. Normally the code that inserts the hook tries to identify whether or not it is possible and provide the developer with a sensible error message, but in edge cases like this you can end up with a low-level error.

Trying to avoid this at the cost of making each and every patching noticeably slower is not really viable approach as it doesn't change the situation (other than producing a better error message (it will still not be possible to patch in a generic hook into such commands).

So I think the only thing that should be done here is to strengthen the documentation and stating that in a few cases one might get low-level errors instead of a decent error message when using a generic hook is not possible.

[ysalmon]

I see this is tricky.

Is there a way to do the same thing as \cs_set_eq:cN { ~ } \space without using this syntax ?
My naïve idea would be \cs_set_eq:NN \ \space but I guess that would have been used if possible.

The definition is using the L3 catcode table, and assumes that the standard tex/L2 catcode table is in force when its execution starts.

[FrankMittelbach]

To be honest I have no idea what this \cs_set_eq:cN { ~ } is trying to achieve or why it shouldn't be possible to use \ ` instead. In your simplified MWE it obviously doesn't make any sense, but that is probably due to other code being removed.

[github-actions]

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