Multicolumn width calculated for lX columns

Question

I have the following table:

\documentclass{article}
\usepackage{tabu, longtable}

\begin{document}
\begin{longtabu} to \textwidth {|l|l|l|lX|}

 1 & Column 2 & 3 & \multicolumn{2}{l|}{Description} \\ \hline
 1 & 2 & 3 & A: & Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum. \\ \hline

 & & & \multicolumn{2}{p{8cm}|}{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum.} \\ \hline

\end{longtabu} 
\end{document}

The last row uses a fixed width for the paragraph (8cm). This does not fit perfectly but shows how the result should look.

My question is: Is it possible to replace the fixed value by something else so Latex calculates the correct width of the last two columns for me?

Answer 1

Actually what I said in comments wasn't quite right tabularx only really knows the widths of the X columns and the total table width. longtable on the other hand does know (and record in the aux file) all the column widths, tabu is using a mixture of the two so we can use the longtable data:

This could be wrapped up in a nicer interface but basically you need to do:

\documentclass{article}
\usepackage{tabu, longtable}

\begin{document}
\makeatletter
\newdimen\zzz
\ifx\LT@i\@undefined
\zzz=8cm
\else
\global\zzz\z@
\begingroup
\@tempcnta\z@
\def\LT@entry#1#2{%
\advance\@tempcnta\@ne
\ifnum\@tempcnta=4 \global\advance\zzz#2\relax\fi
\ifnum\@tempcnta=5 \global\advance\zzz#2\relax\fi
}
\LT@i
\endgroup
\advance\zzz-2\tabcolsep
\advance\zzz-\arrayrulewidth
\begin{longtabu} to \textwidth {|l|l|l|lX|}

 1 & Column 2 & 3 & \multicolumn{2}{l|}{Description} \\ \hline
 1 & 2 & 3 & A: & Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum. \\ \hline

 & & & \multicolumn{2}{l|}{%
\rlap{\parbox[t]{\zzz}{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer eget est id enim vehicula sodales et quis ipsum.}}}\\ \hline

\end{longtabu} 
\end{document}