On 28/03/2016 19:21, Denis Bitouzé wrote:
> Le 26/03/16 à 18h47, Joseph Wright
> <[log in to unmask]>
> a écrit :
> 
>> Hello all,
> 
> Hello Joseph,
> 
>> Submitted today to CTAN, highlights:
>>
>>  - Fix category code production by \tl_lower_case:n, etc., when
>>    Unicode engines are in use
>>  - Produce catcode-11 "e" in \fp_to_scientific:n, etc. (breaking
>>    change)
>>  - Various minor documentation tweaks
>>  - Updates to some accent handling by case changing code
>>  - Preparations for TeX Live 2016 binary changes
> 
> The following M(n)WE:
> 
> --8<---------------cut here---------------start------------->8---
> \documentclass{article}
> \usepackage{xparse}
> \usepackage{amsmath}
> 
> \ExplSyntaxOn
> \NewDocumentCommand \foo { O{} m } {
>   \tl_if_empty:nTF {#1} {%
>     #2
>   }{
>     #2
>   }
> }
> \ExplSyntaxOff
> 
> \begin{document}
> \begin{align*}
>   f & = \foo[a]{a}.
> \end{align*}
> \end{document}
> --8<---------------cut here---------------end--------------->8---
> 
> works like a charm with:
> 
>   ┌────
>   │   xparse.sty    2016/01/19 v6377 L3 Experimental document command parser
>   │    expl3.sty    2016/01/19 v6377 L3 programming layer (loader)
>   │ expl3-code.tex    2016/01/19 v6377 L3 programming layer
>   │ l3pdfmode.def    2015/11/11 v6250 L3 Experimental driver: PDF mode
>   └────
> 
> but fails with:
> 
>   ┌────
>   │    xparse.sty    2016/03/26 v6466 L3 Experimental document command parser
>   │    expl3.sty    2016/03/26 v6466 L3 programming layer (loader)
>   │ expl3-code.tex    2016/03/26 v6466 L3 programming layer
>   │ l3pdfmode.def    2016/03/26 v6465 L3 Experimental driver: PDF mode
>   └────
> 
> with the error:
> 
>   ┌────
>   │ ! Incomplete \ifcase; all text was ignored after line 20.
>   │ <inserted text>
>   │                 \fi
>   │ l.20 \end{align*}
>   │
>   │ ? s
>   │ OK, entering \scrollmode...
>   │ ! Extra \or.
>   │ \c__char_xci_tl ->\or: [\or: [\or: [\or: [\or:
>   │                                                \or: [[\or: [\or: [\or: \or: ...
>   │ l.20 \end{align*}
>   │
>   │ Runaway argument?
>   │ ! Forbidden control sequence found while scanning use of \foo .
>   │ <inserted text>
>   │                 \par
>   │ l.20 \end{align*}
>   │
>   │ ! Missing $ inserted.
>   │ <inserted text>
>   │                 $
>   │ l.20 \end{align*}
>   │
>   │ ! Missing } inserted.
>   │ <inserted text>
>   │                 }
>   │ l.20 \end{align*}
>   │
>   │ ! Emergency stop.
>   │ <recently read> \endtemplate
>   │
>   │ l.20 \end{align*}
>   │
>   │ !  ==> Fatal error occurred, no output PDF file produced!
>   │ Transcript written on test.log.
>   └────
> 
> Sorry for these bad news ;)

Thanks for this: I'm pretty sure I know what the issue is and can
probably have a fix to CTAN later today.

Joseph