Incorrect alignment with \prescript

[quark67]

The bug where described here by other user at 2013: https://tex.stackexchange.com/questions/109694/the-prescript-command-from-the-mathtools-package-gives-incorrect-alignment

The code (taken from the previous site):

\documentclass{article}
\usepackage{amsmath,mathtools}
\begin{document}
$\prescript{14}{2}{\mathbf{C}}^{5+}_{2}$
\end{document}

produce this (red line added to make the difference clearer):

But I found also a bug for the sup argument.

Here is a code:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$\prescript{14}{2}{\mathbf{C}}^{4+}_{2}$
\end{document}

The result (compare the left and right "4": at left, the "4" is slightly too high):

More interesting: if we drop the "+", the sub part (the "2") are correctly aligned, but the left "4" is too hight (or the right "4" too low, but the position of the right part is not controlled by mathtools).

The code:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$\prescript{14}{2}{\mathbf{C}}^{4}_{2}$
\end{document}

The result:

[eg9]

This would need some refinement, but it should give the idea

\documentclass{article}
\usepackage{amsmath}

\NewDocumentCommand{\prescript}{mmme{^_}}{%
  \vphantom{#3}% the base
  ^{\prescriptalign{{#1}{#4}{#2}{#5}}}% the superscript
  _{\prescriptalign{{#2}{#5}{#1}{#4}}}%  the subscript
  \kern-\scriptspace
  #3% the base
  ^{\IfValueT{#4}{#4}\vphantom{#1}}% the superscript
  _{\IfValueT{#5}{#5}\vphantom{#2}}% the subscript
}

\NewDocumentCommand{\prescriptalign}{m}{%
  \mathpalette\prescriptalignaux{#1}%
}
\NewDocumentCommand{\prescriptalignaux}{mm}{%
  \prescriptalignauxi{#1}#2%
}
\ExplSyntaxOn
\NewDocumentCommand{\prescriptalignauxi}{mmmmm}
 {
  \hbox_set:Nn \l_tmpa_box { \mathsurround0pt $#1#2\IfValueT{#3}{\vphantom{#3}}$ }
  \hbox_set:Nn \l_tmpb_box { \mathsurround0pt $#1#4\IfValueT{#5}{\vphantom{#5}}$ }
  \makebox
    [\dim_max:nn { \box_wd:N \l_tmpa_box } { \box_wd:N \l_tmpb_box } ]
    [r]
    { \box_use:N \l_tmpa_box }
 }
\ExplSyntaxOff

\def\baserule{\makebox[0pt]{\vrule height 0pt depth 0.1pt width 10cm}}

\begin{document}

\begin{gather*}
\prescript{14\baserule}{2\baserule}{\mathbf{C}}^{5+}_{2}
\\
\prescript{14\baserule}{2\baserule}{\mathbf{C}}^{5}_{2}
\\
\prescript{14\baserule}{2\baserule}{\mathbf{C}}_{2}
\\
\prescript{14\baserule}{2\baserule}{\mathbf{C}}^{5+}
\end{gather*}

\end{document}

[daleif]

But is it now considered save to start using latex3 syntax in mathtools? It would be nice, but we have no idea how many people are updating these by hand. I all for drawing a line at some point and declaring that from now on we expect users to use an kernel newer than X