Why does *[\fill] not work in \endlastfoot of longtable?

Question

According to this it is possible to put at least one element between \endhead and \endlastfoot on automatic pagebreak by putting the last element into the \endlastfoot. But if I also want to have the actual endlastfoot at the buttom of the page, the *[\fill] does not work like it would between the last element and the endlastfoot.

\documentclass[10pt, a4paper]{article}
\usepackage{longtable}
\pagestyle{empty}
\begin{document}
\vspace*{17.4cm}
\begin{longtable}{@{}lc@{}}
Date &       Grade
\endhead
%
2016-01-15 & C\ *[\fill]
\bfseries Summary:
\endlastfoot
%
2016-01-01 & A\
2016-01-15 & B\
2016-01-01 & A\
2016-01-15 & B\
2016-01-01 & A\
\end{longtable}
\end{document}

Answer 1

The longtable syntax here matches the syntax of tabular (in fact it is just using the under lying tabular implementation) \\[...] (normally) adds a strut (zero width rule) with the specified depth to force the row to have greater depth. A rule dimension is always a fixed length and will take the natura length if a stretchy length is supplied, which is 0pt here. In some cases \\[...] adds a vertical skip rather than a rule in which case a stretch space might do something, but thelongtable foot is a box that is inserted so always has fixed lengths,

Answer 2

Interestingly, I couldn't even use \rule in the lastfoot to add extra space. So I split it into two tabulars.

\documentclass[10pt, a4paper]{article}
\usepackage{longtable}
\pagestyle{empty}
\usepackage{showframe}
\begin{document}
\vspace*{17.4cm}
\begin{longtable}{@{}lc@{}}
Date &       Grade
\endhead
2016-01-15 & C \
\phantom{\bfseries Summary:}
\endlastfoot
2016-01-01 & A\
2016-01-15 & B\
2016-01-01 & A\
2016-01-15 & B\
2016-01-01 & A\
\end{longtable}
\vfill
\noindent\hfil
\begin{tabular}[b]{@{}lc@{}}
\phantom{2016-01-15} & \phantom{Grade}\
\bfseries Summary:
\end{tabular}
\end{document}

---

It is possible to get all the column widths from the aux file instead of using \phantom. See here. By using two longtables one can share widths. The downside is that to shrink the widths one has to delete the old aux file.

\documentclass[10pt, a4paper]{article}
\usepackage{longtable}
\pagestyle{empty}
\usepackage{showframe}
\newlength{\LTwidth}
\newcommand{\LTwidthA}{0pt}
\newcommand{\LTwidthB}{0pt}
\makeatletter
\newcommand*{\getLTwidth}[1][0]% #1 = offet (optional): 0 = previous, 1 = next
{\bgroup% for \count1 and \LT@entry
  \count1=#1\relax
  \advance\count1 by \value{LT@tables}%
  @ifundefined{LT@@roman{\count1}}{\LTwidth=\textwidth}{%
    \dimen0=0pt%
    \count2=0
    \def\LT@entry##1##2{\advance\dimen0 by ##2\relax
      \advance\count2 by 1
      \expandafter\xdef\csname LTwidth@Alph{\count2}\endcsname{##2}}
    \csname LT@@roman{\count1}\endcsname
  }%
  \global\LTwidth=\dimen0
\egroup}
\makeatother
\newcommand{\maxcell}[3]% #1 = width, #2 = l/r/c, #3 = text
{\bgroup
  \dimen0=#1\relax
  \sbox0{#3}%
  \ifdim\wd0>\dimen0
    \usebox0
  \else
    \makebox[\dimen0][#2]{\usebox0}%
  \fi
\egroup}
\begin{document}
\vspace*{17.8cm}
\getLTwidth[2]% get widths from second table
\begin{longtable}{@{}lc@{}}
Date &       Grade
\endhead
2016-01-15 & C
\endlastfoot
\maxcell{\dimexpr \LTwidthA-\tabcolsep}{l}{2016-01-01} &
\maxcell{\dimexpr \LTwidthB-\tabcolsep}{c}{A} \% any row will do
2016-01-15 & B\
2016-01-01 & A\
2016-01-15 & B\
2016-01-01 & A\
\end{longtable}
\getLTwidth% get widths from previous table
\vfill
\noindent\hfil
\begin{longtable}{@{}lc@{}}
  %\rule{\LTwidthA}{1pt}\hspace{-\tabcolsep} & \hspace{-\tabcolsep}\rule[1pt]{\LTwidthB}{1pt} \
  \maxcell{\dimexpr \LTwidthA-\tabcolsep}{l}{\bfseries Summary:} &
  \maxcell{\dimexpr \LTwidthB-\tabcolsep}{c}{ }
\end{longtable}\unskip% remove gap at bottom
\end{document}