\selectlanguage{spanish} in preamble causes errors

[tomh-overleaf]

Minimal Example

\documentclass{article}

\usepackage[spanish]{babel}

\selectlanguage{spanish}

\begin{document}

Hello, world!

\end{document}

Error message

! Incomplete \iffalse; all text was ignored after line 129.
<inserted text> 
                \fi 
l.7 \begin{document}
                    ^^M

Listfiles

 *File List*
 article.cls    2023/05/17 v1.4n Standard LaTeX document class
  size10.clo    2023/05/17 v1.4n Standard LaTeX file (size option)
   babel.sty    2023/07/09 v3.91 The Babel package
 spanish.ldf
babel-spanish.tex
l3backend-pdftex.def    2023-04-19 L3 backend support: PDF output (pdfTeX)
 ***********

Observations

• Removing \selectlanguage{spanish} clears the error.
• Manually getting an older babel, e.g. 2022/07/20 3.78, clears the error.
• Replacing spanish with e.g. czech clears the error.
• Loading babel as e.g. \usepackage[spanish,czech]{babel} clears the error.
• However, with \usepackage[czech,spanish]{babel} the error is preserved.

[josephwright]

I think it's a file loading issue: if I manually force expl3 loading in the preamble it works, so there is an issue loading the backend code.

[josephwright]

I see the issue: the catcodes are being activated in begindocument/before, which is before we load l3backend, which then chokes. Thinking where the solution is.

[jbezos]

I’m preparing a new version fixing that.

[josephwright]

@jbezos I think the activation is in the wrong hook, but I'm not sure why begindocument/before was chosen.

[jbezos]

@josephwright See #240. The problem was \normalsfcodes, so I moved \selectlanguage before, but that was clearly wrong (even if all my tests passed). I’ve restored the place where the language was selected (which is the correct one) and devised another fix.

@tomh-overleaf I’ll upload a new version to CTAN very likely in a few hours.

[tomh-overleaf]

@tomh-overleaf I’ll upload a new version to CTAN very likely in a few hours.

Thanks, Javier! We'll pull it next week :)