Can't redefine \val from polyglossia

Question

At some recent package update (not sure which one), polyglossia seems to define \val to contain the value given to the variant key.

In one of my documents I have defined \val, which I don't want to rename because I used it several times in my document. But polyglossia override my definition of \val. I tried with \AtBeginDocument but this doesn't work.

\documentclass{article}

\usepackage{polyglossia}
\setmainlanguage[variant=british]{english}

\show\val

\AtBeginDocument{
    \DeclareDocumentCommand{\val}{}{some new definition} % Does not work
}
\begin{document}
    \val % prints british
\end{document}

Answer 1

You're in a tough situation, which stems from history.

The few examples in the documentation of xkeyval for \define@choicekey all use [\val\nr] and this has instilled the idea that these are mandatory tokens for this case.

They aren't. Any two control sequences are allowed and they are used as scratch macros. Package writers should use their own control sequences; so the example in the documentation

\define@choicekey*{fam}{align}[\val\nr]{left,center,right}{%
  \ifcase\nr\relax
    \raggedright
  \or
    \centering
  \or
    \raggedleft
  \fi
}

adapted to the package foo should better be

\define@choicekey*{fam}{align}[\foo@val\foo@nr]{left,center,right}{%
  \ifcase\foo@nr\relax
    \raggedright
  \or
    \centering
  \or
    \raggedleft
  \fi
}

and the code in the polyglossia definition files should be

\define@choicekey*+{welsh}{date}[\xpg@val\xpg@nr]{long,short}[short]{%
   \ifcase\xpg@nr\relax
      % long:
      \welsh@formaldatetrue
   \or
      % accented:
      \welsh@formaldatefalse
   \fi
   \xpg@info{Option: Welsh, date=\xpg@val}%
}{\xpg@warning{Unknown date value `#1'}}

Unfortunately, most packages or classes using xkeyval and choice keys adopted \var\nr. These scratch control sequences are not set inside a group, so any time one performs a choice key using them they are redefined and their final value is unpredictable.

What can you do? Avoid \val as the name of a personal macro.

I filed a issue report for polyglossia and I'll do what's possible to inform authors of other packages. But it will take long until the situation improves.