amsmath: column issues with aligned, alignedat and gathered

[nbeisert]

Brief outline of the bug

There are a couple of minor inconsistencies with the aligned, alignedat and gathered environments of amsmath (which likely do not surface prominently in actual documents):

• gathered uses \endaligned

  latex2e/required/amsmath/amsmath.dtx

  Line 4026 in 0e10f72

  \endaligned

  which invokes \restorecolumn@

  latex2e/required/amsmath/amsmath.dtx

  Line 3976 in 0e10f72

  \restorecolumn@

  but there is no corresponding \savecolumn@ as in aligned

  latex2e/required/amsmath/amsmath.dtx

  Line 3865 in 0e10f72

  \savecolumn@ % Assumption: called inside a group

  This may mess up the proper counting of columns in parent alignment structures.
• alignedat terminates processing of the body by \crcr instead of by \math@cr@@@

  latex2e/required/amsmath/amsmath.dtx

  Line 3975 in 0e10f72

  \crcr\egroup

  Effectively, this omits the check of maximum column on the final row. This also omits the elimination of the undesired trailing \tabsep in aligned on the final row.
• \math@cr@@@aligned checks whether the column counter \@column is even or odd, but it never resets it:

  latex2e/required/amsmath/amsmath.dtx

  Line 3769 in 0e10f72

  \def\math@cr@@@aligned{%

  The corresponding \math@cr@@@alignedat performs a reset:

  latex2e/required/amsmath/amsmath.dtx

  Line 3945 in 0e10f72

  \global\column@\z@

  Effectively, this sometimes leaves the undesired trailing \tabsep in place for aligned.

Minimal example showing the bug

\RequirePackage{latexbug}
\documentclass{article}
\usepackage{amsmath}

\makeatletter
\def\@amsmath@err#1#2{\quad\text{ERROR}}

\begin{document}

detection of excessive columns works in nesting here:
\[
\begin{alignedat}{1}
YYYYY
&\quad\text{col}: \the\column@
\\
\begin{aligned}Y&Y&Y&Y&Y\end{aligned}
&\quad\text{col}: \the\column@/\the\maxfields@
&\quad\text{col}: \the\column@/\the\maxfields@
\\
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
\end{alignedat}
\]

detection of excessive columns can be misled by gathered:
\[
\begin{alignedat}{1}
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
\\
\begin{gathered}YYYYY\end{gathered}
&\quad\text{col}: \the\column@/\the\maxfields@
&\quad\text{col}: \the\column@/\the\maxfields@
\\
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
\end{alignedat}
\]

alignedat does not detect excessive column in final row:
\[\begin{alignedat}{1}
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
&\quad\text{col}: \the\column@/\the\maxfields@
\\
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
&\quad\text{col}: \the\column@/\the\maxfields@
\\
YYYYY
&\quad\text{col}: \the\column@/\the\maxfields@
&\quad\text{col}: \the\column@/\the\maxfields@
\end{alignedat}\]

Inconsistencies in eliminating the trailing tabsep in aligned:
\[\vrule\begin{aligned}
X\\
X&Y
\end{aligned}\vrule\]
\[\vrule\begin{aligned}
X\\
X&Y\\
\end{aligned}\vrule\]
\[\vrule\begin{aligned}
X&Y
\end{aligned}\vrule\]
\[\vrule\begin{aligned}
X&Y\\
\end{aligned}\vrule\]
\[\vrule\begin{aligned}
X&Y\\
X&Y
\end{aligned}\vrule\]
\[\vrule\begin{aligned}
X&Y\\
X&Y\\
\end{aligned}\vrule\]

\end{document}

[nbeisert]

log and pdf:
bug3b.log
bug3b.pdf