What causes this strange interaction between glossaries and amsmath?

Question

MWE: greatly trimmed from a large document; XeLaTeX; full TeXLive2012 completely up-to-date:

\documentclass{memoir}
\usepackage{fixltx2e}
\usepackage{amsmath}
\usepackage{mathspec}
\usepackage{glossaries}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\end{document}

Fails with "! Package mathspec Error: 'amsmath' must be loaded earlier than `mathspec'."

Now, if I eliminate glossaries, or move it above mathspec in the file, things work. But it isn't very convenient to have to sort ones packages whenever anything gets a new dependency. And in any case, I do load amsmath before mathspec, don't I?

Really puzzled by this one. Any illumination would be most welcome.

(BTW, not strictly MWE, as it also fails with article.

Answer 1

The problem is that one of the subpackages loaded by datatool, namely datatool-base has the line

\RequirePackage{amsmath}

and mathspec sets up a (very poor) check for possible loading of amsmath after it, which triggers the error even if amsmath has already been loaded when another package requires it.

The normal setup of LaTeX is not reloading packages, so the call by datatool-base should be completely harmless, as far as mathspec is concerned. This is undoubtedly a bug.

---

The package mathspec says

\let\original@RequirePackage\RequirePackage
\renewcommand\RequirePackage[2][]{
  \ifstrequal{#2}{amsmath}
    {\PackageError{mathspec}
       {`amsmath' must be loaded earlier than `mathspec'}
       {Edit the document so that `amsmath' is required earlier than `mathspec'.}}
    {\relax}
  \original@RequirePackage[#1]{#2}}
\@onlypreamble\RequirePackage
\let\usepackage\RequirePackage
\@onlypreamble\usepackage

which is quite bad code, in my opinion: \RequirePackage is redefined to check if its mandatory argument is exactly amsmath. So, if one says

\usepackage{mathspec}
\usepackage{amsmath,amsthm}

the check will fail and amsmath will be happily loaded after mathspec.

Judging from the comments in the package, the only reason why amsmath should be loaded before mathspec is a problem with the definition of \varTheta. This can be solved much more easily by changing lines 103–106, which read

103 \ifdef{\varTheta}
104   {\let\eu@cm@varTheta\varTheta}
105   {\relax}
106 \let\varTheta\Theta

into

\AtBeginDocument{
  \ifdef{\varTheta}
    {\let\eu@cm@varTheta\varTheta}
    {}
\let\varTheta\Theta
}

and removing the final lines with the redefinition of \RequirePackage.

---

A workaround until the mathspec package is fixed:

\documentclass{memoir}
\usepackage{fixltx2e}
\usepackage{amsmath}
\usepackage{mathspec}

\makeatletter % undo the wrong changes made by mathspec
\let\RequirePackage\original@RequirePackage
\let\usepackage\RequirePackage
\makeatother

\usepackage{glossaries}
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\end{document}