array.dtx: Problems with \firsthline and \lasthline

[RuixiZhang42]

Brief outline of the bug

\hline adds a horizontal rule, nothing more. And \firsthline and \lasthline should behave the same. (I’m using an older format LaTeX2e <2019-10-01> patch level 3, but the current issue still stands)

Minimal example showing the bug

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}
\usepackage{array}
\begin{document}

\setbox0=\hbox{%
  \begin{tabular}{@{}c@{}c@{}}
  \hline
  &\null\hskip10pt\relax\null\\
  \hline
  \end{tabular}%
}%
\copy0 %
With \verb|\hline|.\\
Actual width (in sp unit):
\number\wd0 \\ % as expected
Expected:
1st column 0\,sp +
2nd column 1\,sp (guard) + 655360\,sp (content).

\medskip

\setbox0=\hbox{%
  \begin{tabular}{@{}c@{}c@{}}
  \firsthline
  &\null\hskip10pt\relax\null\\
  \hline
  \end{tabular}%
}%
\copy0 %
With \verb|\firsthline|.\\
Actual width (in sp unit):
\number\wd0 \ ???\\
This turns out to be
\verb|2*\tabcolsep| (\the\numexpr\tabcolsep*2\relax\,sp) +
1\,sp + 1\,sp + 655360\,sp.

\medskip

There are two problems:
(1)~\verb|\firsthline| should suppress \verb|\tabcolsep| on two sides;
(2)~\verb|\firsthline| should NOT produce any extra guard (1\,sp).

\end{document}

Log file (required) and possibly PDF file

first-last-hline.log

Proposed fixes

The fixes are straightforward: Use @{} to suppress unintentional \tabcolsep, and use \unskip to cancel the 1sp guard.

Therefore,

latex2e/required/tools/array.dtx

Lines 2921 to 2922 in 59b3e83

	\newcommand{\firsthline}{%
	\multicolumn1c{%

becomes

\newcommand{\firsthline}{%
  \multicolumn1{@{}c@{}}{\unskip

Furthermore,

latex2e/required/tools/array.dtx

Line 2962 in 59b3e83

\newcommand{\lasthline}{\hline\multicolumn1c{%

becomes

\newcommand{\lasthline}{\hline\multicolumn1{@{}c@{}}{\unskip