a "... doesn't match its definition" problem, that I don't understand

Question

I managed to extract from my code the following MnWE which shows the problem i encounter :

Example 1

\begin{document}
\newcommand{\createDATA}[1]{
  \xdef\DATA{}
  \newcommand{\test}[2][]{
    \xdef\DATA{\DATA args=##1 , ##2 }
  }
  \test[x]{\macom[x]{blabla}}
}
\newcommand{\macom}[2][q]{AAA}
\createDATA
\DATA
\end{document}

The compilation gives an error : ! Use of \\test doesn't match its definition. which I can't understand. On the other hand, notice that if in the definition of \createDATA I replace \macom by a command without optional argument, then it works :

Example 2

\begin{document}
\newcommand{\createDATA}[1]{
  \xdef\DATA{}
  \newcommand{\test}[2][]{
    \xdef\DATA{\DATA args=##1 , ##2 }
  }
  \test[x]{\macomm{blabla}}
}
\newcommand{\macomm}[1]{BBB}
\createDATA
\DATA
\end{document}

It is very mysterious to me...

Answer 1

You are trying to \edef (\xdef = \global\edef) the macro \macom. However, as \macom is a command with an optional argument, this cannot be done safely. You need to either not do an \edef, use the LaTeX protected mechanism:

\documentclass{article}
\begin{document}
\makeatletter
\newcommand{\createDATA}[1]{%
  \xdef\DATA{}%
  \newcommand{\test}[2][]{%
    \protected@xdef\DATA{\DATA args=##1 , ##2 }%
  }%
  \test[x]{\macom[x]{blabla}}%
}
\makeatother
\newcommand{\macom}[2][q]{AAA}
\createDATA
\DATA
\end{document}

or make \macrom engine robust using the e-TeX extensions, for example

\documentclass{article}
\usepackage{etoolbox}
\begin{document}
\newcommand{\createDATA}[1]{%
  \xdef\DATA{}%
  \newcommand{\test}[2][]{%
    \xdef\DATA{\DATA args=##1 , ##2 }%
  }%
  \test[x]{\macom[x]{blabla}}%
}
\newrobustcmd{\macom}[2][q]{AAA}
\createDATA
\DATA
\end{document}

(By the way, you have a lot of extraneous spaces in your commands: I've added % to kill line ends and deal with these.)

For more on what robust, protected and 'fragile' means, see What is the difference between Fragile and Robust commands?

---

The reason that you cannot \edef a command with optional arguments is that these are picked up using the TeX primitive \futurelet. This performs an assignment, and assignments are not expandable. On the other hand, grabbing a mandatory argument does not require any tricks, so it is expandable.

[It's a bit more complex than that, as there are ways of expandably testing for optional arguments. These are more restrictive than the non-expandable approach, and the LaTeX2e kernel therefore goes with comprehensive-but-non-expandable. For pre-constructed methods for both expandable and non-expandable optional argument grabbing, with automatically robust functions and lots of additional cleverness, see the LaTeX3 xparse module.]