Update fancyhdr.sty

[TerraAr]

Bug fixes.

Added package needed for command \NewDocumentCommand in old line 720

[pietvo]

How old is your LaTeX version?

[FrankMittelbach]

Older than 2020 I would say. xparse was added to core LaTeX in 2020-10-01 release (see texdoc ltnews). I don't think xparse should be loaded. If at all, I would suggest adding a \NeedsTeXFormat{LaTeX}[2020-10-01] in the package.

[pietvo]

@FrankMittelbach
Or I could load it conditionally, like

\ifdefined\NewDocumentCommand\else\RequirePackage{xparse}\fi

Until now I have tried to support older LaTeX versions, if they are not excessively old.
Could there be a problem with using \RequirePackage inside an \if?

[FrankMittelbach]

@FrankMittelbach Or I could load it conditionally, like

\ifdefined\NewDocumentCommand\else\RequirePackage{xparse}\fi

Until now I have tried to support older LaTeX versions, if they are not excessively old. Could there be a problem with using \RequirePackage inside an \if?

please don't. That leads to really messy code and overall it is a maintenance nightmare. It is ok to try to support a couple of releases back, but in my opinion the limit should be low. Otherwise you introduce all kind of dependencies with other packages that then have to behave conditionally based on what you do etc.

In my opinion 2020 or even 2022 is a clear cutoff point, because so much new stuff got added to LaTeX in the recent years.

It adds a lot of burden to the few people that keep LaTeX alive and kicking if users run with really old versions but expect to add packages that have been written with features in mind that were provided many years later.

but if you want conditional processing I would organize that by LaTeX release, ie

\IfLaTeXFormatAtLeastTF{2020-10-01}{true}{false}