Optional argument problem

Question

This example works but if I try to put an optional argument changing

\newcommand\TBLdiag[5]{%

in

\newcommand\TBLdiag[5][\tabcolsep]{%

it does not work anymore. Of course I call \TBLdiag with only four arguments then.

\documentclass[10pt,french]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage[locale=FR]{siunitx}
\usepackage{array}
\usepackage{calc}
\usepackage{tikz,fourier,ragged2e}

\newcommand\TBLdiag[5]{%
    \multicolumn{1}{|m{#2}|}{%
    \hskip-\tabcolsep
    \begin{tikzpicture}[baseline=0,anchor=south west,outer sep=0]
        \path[use as bounding box] (0,0)
            rectangle (#2+2\tabcolsep,\baselineskip);
        \node[minimum width={#2+2\tabcolsep-\pgflinewidth},
            minimum  height=\baselineskip+#3-\pgflinewidth]
            (box) {};
        \draw[line cap=round]
            (box.north west) -- (box.south east);
        \node[anchor=south west,align=left,inner sep=#1]
            at (box.south west) {#4};
        \node[anchor=north east,align=right,inner sep=#1]
            at (box.north east) {#5};
        \end{tikzpicture}%
    \rule{0pt}{\baselineskip+#3-\pgflinewidth}%
    \vspace{-.3\baselineskip}%\hskip-\tabcolsep
    }
}

\begin{document}
\begin{tabular}{%
    |>{\Centering}m{4.5cm}
    |*{4}{>{\Centering}m{2.5em}|}}\hline

\TBLdiag{\tabcolsep}{4.5cm}{3.4em}{Coefficient\\ de frottement $c$}{Vitesse $V$\\ (en km/h)}&50&90&110&130\\ \hline
$c=\num{0,8}$\newline route s\`eche, bitume neuf &&&&\\ \hline
$c=\num{0,7}$\newline route s\`eche, bitume moyen &&&&\\ \hline
$c=\num{0,3}$\newline route mouill\'ee, bitume moyen &&&&\\ \hline
\end{tabular}

\end{document}

Thanks to Steven B. Segletes there is a minimal example that works / doesn't works :

\documentclass{article}
\usepackage{array}

\newcommand\TBLdiag[2]{% WORKS
%\newcommand\TBLdiag[2][]{% DOESN'T WORK     
\multicolumn{1}{|m{3.4em}|}{X} }

\begin{document}
\begin{tabular}{|l|c|c|c|c|}\hline
\TBLdiag{0pt}{4.5cm}&50&90&110&130\\ \hline% WORKS
%\TBLdiag[0pt]{4.5cm}&50&90&110&130\\ \hline% DOESN'T WORK
\end{tabular}
\end{document}

Answer 1

\multicolumn can never be hidden in a command defined with an optional argument with \newcommand. This has been discussed before: TeX needs to expand macros in order to see whether \multicolumn is present, but with such a command it can't find it in time.

You can do with xparse; I know that the second example is wrong and is just to see that the optional argument works. But of course this is just for showing how you could do; please, avoid tables constructed in that way: the diagonal box is simply horrible.

\documentclass[10pt,french]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage[locale=FR]{siunitx}
\usepackage{array}
\usepackage{calc}
\usepackage{tikz,fourier,ragged2e,xparse}

\DeclareExpandableDocumentCommand\TBLdiag{O{\tabcolsep}mmmm}{%
    \multicolumn{1}{|m{#2}|}{%
    \hskip-#1
    \begin{tikzpicture}[baseline=0,anchor=south west,outer sep=0]
        \path[use as bounding box] (0,0)
            rectangle (#2+2\tabcolsep,\baselineskip);
        \node[minimum width={#2+2\tabcolsep-\pgflinewidth},
            minimum  height=\baselineskip+#3-\pgflinewidth]
            (box) {};
        \draw[line cap=round]
            (box.north west) -- (box.south east);
        \node[anchor=south west,align=left,inner sep=#1]
            at (box.south west) {#4};
        \node[anchor=north east,align=right,inner sep=#1]
            at (box.north east) {#5};
        \end{tikzpicture}%
    \rule{0pt}{\baselineskip+#3-\pgflinewidth}%
    \vspace{-.3\baselineskip}%\hskip-\tabcolsep
    }
}

\begin{document}
\begin{tabular}{%
    |>{\Centering}m{4.5cm}
    |*{4}{>{\Centering}m{2.5em}|}}\hline
\TBLdiag{4.5cm}{3.4em}{Coefficient\\ de frottement $c$}{Vitesse $V$\\ (en km/h)}&50&90&110&130\\ \hline
$c=\num{0,8}$\newline route s\`eche, bitume neuf &&&&\\ \hline
$c=\num{0,7}$\newline route s\`eche, bitume moyen &&&&\\ \hline
$c=\num{0,3}$\newline route mouill\'ee, bitume moyen &&&&\\ \hline
\end{tabular}

\begin{tabular}{%
    |>{\Centering}m{4.5cm}
    |*{4}{>{\Centering}m{2.5em}|}}\hline
\TBLdiag[2\tabcolsep]{4.5cm}{3.4em}{Coefficient\\ de frottement $c$}{Vitesse $V$\\ (en km/h)}&50&90&110&130\\ \hline
$c=\num{0,8}$\newline route s\`eche, bitume neuf &&&&\\ \hline
$c=\num{0,7}$\newline route s\`eche, bitume moyen &&&&\\ \hline
$c=\num{0,3}$\newline route mouill\'ee, bitume moyen &&&&\\ \hline
\end{tabular}

\end{document}

Here's how I would typeset the table:

\documentclass[10pt,french]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage[locale=FR]{siunitx}
\usepackage{array}
\usepackage{fourier,booktabs}

\newcommand{\smashedcell}[1]{%
  \smash{\begin{tabular}[b]{@{}c@{}}\strut#1\strut\end{tabular}}%
}

\begin{document}
\begin{tabular}{%
    >{\centering}m{4.5cm}
    *{4}{>{\centering\arraybackslash}m{2.5em}}}
\toprule
& \multicolumn{4}{c}{Vitesse $V$ (en \si{km/h})} \\
\cmidrule{2-5}
\multicolumn{1}{c}{\smashedcell{Coefficient\\ de frottement $c$}} &50&90&110&130\\ 
\midrule
$c=\num{0,8}$\\ route s\`eche, bitume neuf &&&&\\
\midrule
$c=\num{0,7}$\\ route s\`eche, bitume moyen &&&&\\
\midrule
$c=\num{0,3}$\\ route mouill\'ee, bitume moyen &&&&\\
\bottomrule
\end{tabular}

\end{document}