error with \fpeval in p-columntype if array.sty is loaded

[u-fischer]

If \fpeval is used (directly or indirectly through a command) in a p-column this can error in some tabular preambles. Two examples are below.

\RequirePackage{latexbug}
\documentclass{article}
\usepackage{array}
\begin{document}
\begin{tabular}{p{\fpeval{15}pt}p{\fpeval{15}pt}p{\fpeval{15}pt}p{\fpeval{15}pt}}
a
\end{tabular}


\begin{tabular}{p{\fpeval{15}pt}lll}
a
\end{tabular}

\end{document}

! Extra \or.
\@mkpream ...ss }\ifcase \@lastchclass \@acol \or 
                                                  \or \@acol \or \@preamerr ...
l.33 ...al{15}pt}p{\fpeval{15}pt}p{\fpeval{15}pt}}

Log file (required) and possibly PDF file

test-utf8.log

A suggested fix is to add a \dimexpr (this would also allow direct calculations in the p-columntype, but must be checked for compability with the calc package.

\def\@mkpream#1{\gdef\@preamble{}\@lastchclass 4 \@firstamptrue
   \let\@sharp\relax
   \def\@startpbox##1{\unexpanded\expandafter{\expandafter
                      \@startpbox\expandafter{\expandafter\dimexpr##1}}}\let\@endpbox\relax %<-----------
   \let\do@row@strut\relax
   \let\ar@align@mcell\relax
   \@temptokena{#1}\@tempswatrue
   \@whilesw\if@tempswa\fi{\@tempswafalse\the\NC@list}%
   \count@\m@ne
   \let\the@toks\relax
   \prepnext@tok
   \expandafter \@tfor \expandafter \@nextchar
    \expandafter :\expandafter =\the\@temptokena \do
   {\@testpach
   \ifcase \@chclass \@classz \or \@classi \or \@classii
     \or \save@decl \or \or \@classv \or \@classvi
     \or \@classvii \or \@classviii
     \or \@classx
     \or \@classx \fi
   \@lastchclass\@chclass}%
   \ifcase\@lastchclass
   \@acol \or
   \or
   \@acol \or
   \@preamerr \thr@@ \or
   \@preamerr \tw@ \@addtopreamble\@sharp \or
   \or
   \else  \@preamerr \@ne \fi
   \def\the@toks{\the\toks}}