On 16/07/2015 20:47, David Carlisle wrote:
> A simplified example, should anyone be looking at this is:
> 
> \listfiles
> \documentclass{article}
> \usepackage{xparse}
> \DeclareDocumentCommand\foo{mO{xxx}}{[#1][#2]}
> \usepackage{array}
> \begin{document}
> a $x$
> 
> \tracingmacros2
> 
> %\begin{tabular}{>{\foo{zz}[]}l}% works
> \begin{tabular}{>{\foo{zz}}l}% fails
>   text
> \end{tabular}
> \end{document}

I think I know where to look at this but not quite what is up! I guess
as this is linked to xparse it's one for me: I'll log on GitHub and
investigate.
--
Joseph Wright