Specifying minimal (not fixed) width in tabular

See ADDENDUM and BONUS

First, the original approach.

Here is a \newcolumntype solution, except that, for a reason that I should know but I don't, it won't work in the last column of the tabular. I'm sure David C. could tell me off the cuff why.

So I've provided C{}, L{}, and R{} column types for specifying a minimum width, and an Q column type to use for a phantom, to get around the deficiency I initially cited.

\documentclass{article}
\usepackage{tabularx,stackengine}
\newcolumntype{L}[1]{>{\minwd l{#1}}l<{\endminwd}}
\newcolumntype{C}[1]{>{\minwd c{#1}}c<{\endminwd}}
\newcolumntype{R}[1]{>{\minwd r{#1}}r<{\endminwd}}
\def\minwd#1#2#3\endminwd{\stackengine{0pt}{#3}{\rule{#2}{0pt}}{O}{#1}{F}{F}{L}}
\newcolumntype{Q}{@{}c@{}}
\begin{document}
\begin{tabular}{|C{1.2cm}|C{1.2cm}|R{2cm}|l|}
a & a& b& c\\
aaaaaaaaaaaaaaaaa& aa& bb& cc\\
\end{tabular}
\bigskip


\begin{tabular}{|C{1.2cm}|C{1.2cm}|R{2cm}|L{1.5cm}|Q}
a & a& b& c&\\
aaaaaaaaaaaaaaaaa& aa& bb& cc&\\
\end{tabular}
\end{document}

---

ADDENDUM

Apparently the way to overcome the deficiency I noted in my original solution is to use \collectcell:

\documentclass{article}
\usepackage{tabularx,stackengine,collcell}
\let\endminwd\relax
\newcolumntype{L}[1]{>{\collectcell\xminwd l{#1}}l<{\endminwd\endcollectcell}}
\newcolumntype{C}[1]{>{\collectcell\xminwd c{#1}}c<{\endminwd\endcollectcell}}
\newcolumntype{R}[1]{>{\collectcell\xminwd r{#1}}r<{\endminwd\endcollectcell}}
\def\minwd#1#2#3\endminwd{\stackengine{0pt}{#3}{\rule{#2}{0pt}}{O}{#1}{F}{F}{L}}
\newcommand\xminwd[1]{\minwd#1}
\begin{document}
\begin{tabular}{|C{1.2cm}|C{1.2cm}|R{2cm}|L{1.5cm}|}
a & a& b& c\\
aaaaaaaaaaaaaaaaa& aa& bb& cc\\
\end{tabular}
\end{document}

And just for fun, if you set the vertical measure of the \rule to something nonzero, say 1pt, you can see how the rule comes into play:

---

BONUS

I realized the technique could be used for creating a D column for aligning a specified number of trailing decimal places. EDITED to handle decimal free inputs.

\documentclass{article}
\usepackage{tabularx,stackengine,collcell}
\let\endecm\relax
\newcolumntype{D}[1]{>{\collectcell\Decm l{#1}}r<{..\endDecm\endcollectcell}}
\def\decm#1#2#3.#4.#5\endDecm{#3.\stackengine{0pt}{#4}{\rule{#2\wd0}{0pt}}{O}{#1}{F}{F}{L}}
\newcommand\Decm[1]{\setbox0=\hbox{0}\decm#1}
\begin{document}
\begin{tabular}{|D{3}|D{2}|D{4}|}
10.345 & 410.34 & 1310.34\\
10 & 11 & 12.\\
210.34 & 1210.3 & 310.3456\\
\end{tabular}
\end{document}

This could be done by placing a row of 0pt \rules with a negative row separator (for proper vertical alignment).

In the example, columns 1 and 2 have minimum width 2cm, while columns 3 and 4 have minimum width 1cm.

\documentclass{article}

\begin{document}
\[
\begin{tabular}{cccc}
\rule{2cm}{0pt}&\rule{2cm}{0pt}&\rule{1cm}{0pt}&\rule{1cm}{0pt}\\[-\arraystretch\normalbaselineskip]]
Longer entry here&2&3&4\\
5&6&Longer entry here&8
\end{tabular}
\]
\end{document}

Update: By placing the \rule in a macro, you can adjust the fixed minimum in the document body similarly to a column type.

Here is the code:

\documentclass{article}

\newcommand{\fixedmin}[1]{\rule{#1}{0pt}&\rule{#1}{0pt}&\rule{#1}{0pt}&\rule{#1}{0pt}\\[-\arraystretch\normalbaselineskip]}

\begin{document}

\noindent Here is a table with fixed minimum width 2 cm.
\[
\begin{tabular}{cccc}\fixedmin{2cm}
Longer entry here&2&3&4\\
5&6&Longer entry here&8
\end{tabular}
\]

\noindent Here is a table with fixed minimum width 1 cm.
\[
\begin{tabular}{cccc}\fixedmin{1cm}
Longer entry here&2&3&4\\
5&6&Longer entry here&8
\end{tabular}
\]

\noindent Here is a table with no fixed minimum width.
\[
\begin{tabular}{cccc}
Longer entry here&2&3&4\\
5&6&Longer entry here&8
\end{tabular}
\]

\end{document}

Here's a way to do it using the collcell package. The idea is to measure the content of the cell and then add rules to make up the deficit where appropriate. I've made versions for c, r and l columns. The vertical lines are just there to show the fixed width columns.

\documentclass{article}
\usepackage{array,calc,collcell}
\newlength{\fwlen}
\newcolumntype{C}[1]{>{\setlength{\fwlen}{#1}\collectcell\fixedCwidth}c<{\endcollectcell}}
\newcolumntype{R}[1]{>{\setlength{\fwlen}{#1}\collectcell\fixedRwidth}r<{\endcollectcell}}
\newcolumntype{L}[1]{>{\setlength{\fwlen}{#1}\collectcell\fixedLwidth}l<{\endcollectcell}}
\makeatletter
\newcommand{\fixedCwidth}[1]{\setlength{\@tempdima}{\widthof{#1}}\ifdim\@tempdima>\fwlen #1\else\setlength{\@tempdimb}{(\fwlen-\@tempdima)/2}\rule{\@tempdimb}{0pt}#1\rule{\@tempdimb}{0pt}\fi}
\newcommand{\fixedLwidth}[1]{\setlength{\@tempdima}{\widthof{#1}}\ifdim\@tempdima>\fwlen #1\else\setlength{\@tempdimb}{\fwlen-\@tempdima}#1\rule{\@tempdimb}{0pt}\fi}
\newcommand{\fixedRwidth}[1]{\setlength{\@tempdima}{\widthof{#1}}\ifdim\@tempdima>\fwlen #1\else\setlength{\@tempdimb}{\fwlen-\@tempdima}\rule{\@tempdimb}{0pt}#1\fi}
\makeatother
\begin{document}


\begin{tabular}{|C{1cm}|L{1cm}|R{1cm}|C{1cm}|C{1cm}|}
123 &   123 &   123 &   123 & This is a wider column\\
5   &   6   &   6   &   8 & 8
\end{tabular}

\end{document}