\cals@withWidthII{1pt}{5pt}\iftrue
 \PackageInfo{test}{user width: \cals@width}
\else
 \PackageInfo{test}{fail}
\fi

\def\defaultDimen{2pt}
\cals@withWidthII\defaultDimen{0pt}\iftrue
 \PackageInfo{test}{fail}
\else
 \PackageInfo{test}{no width}
\fi

\cals@withWidthII\defaultDimen\relax\iftrue
 \PackageInfo{test}{default width: \cals@width}
\else
 \PackageInfo{test}{fail}
\fi

\cals@withWidthII{0pt}\relax\iftrue
 \PackageInfo{test}{fail}
\else
 \PackageInfo{test}{no width}
\fi

%
% Bug hunting: a space
%
\def\cals@default@cs@width{4pt}
\setbox0=\hbox{\hbox{}%
\cals@withWidthII\cals@default@cs@width\relax\iftrue \fi}
\showbox0