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 ;)
-- 
Denis