+cases does not work in align

[Firestar-Reimu]

This is good

\documentclass{article}
\usepackage{amsmath}

\begin{document}
    \begin{align}
        a_n
        &= (-1)^n \\
        &=
        \begin{cases}
            1, & n\ \text{is even} \\
            -1, & n\ \text{is odd}
        \end{cases}
    \end{align}
\end{document}

This is bad

\documentclass{article}
\usepackage{amsmath}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}

\begin{document}
    \begin{align}
        a_n
        &= (-1)^n \\
        &=
        \begin{+cases}
            1, & n\ \text{is even} \\
            -1, & n\ \text{is odd}
        \end{+cases}
    \end{align}
\end{document}

Extra }, or forgotten $.
<recently read> }
\begin{align} on input line 7 ended by \end{+cases}.
Misplaced alignment tab character &.
\@tempa ->\endgroup &&
...

[Sophanatprime]

Not only is the +cases, but those +matrix environments.

One can simply redefine them by using \RenewDocumentEnvironment (and its friends, in ltcmd or its old name xparse), such as:

\DeclareDocumentEnvironment{+cases}{+O{}b}
  {%
    \left\lbrace\begin{+array}{
      column{1} = {leftsep = 0pt}, column{Z} = {rightsep = 0pt},
      colspec = {ll}, stretch = 1.2, #1
    }#2%
    \end{+array}\right.%
  }
  {}

Maybe it is the environ package that causes this problem (I'm not pretty sure).

[lvjr]

Actually, the latest source code of tabularray doesn't use environ package.

[antshar]

Any updates on this?

UPD: nvm, apparently I'm using an old version where these environments are defined via \NewEnv which messes up \\. Redefining environments with xparse solves the problem. In newer versions of tabularray it's already the case, so it's fixed there and the issue can be closed.

[muzimuzhi]

Duplicate of #143