Is the limits command "\lim" incompatible with XeLaTeX?

Question

everybody.

I have problems with the limits command "\lim" when I compile with XeLaTeX and I use the fontspec package. Let me explain it with one example:

If don't use the fontspec package (because I need to use several True Type Fonts), this code produces no errors:

\documentclass[letterpaper,reqno]{book}
\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}
%\usepackage{fontspec}
\usepackage{amssymb,amsmath,amsthm}

\begin{document}
$\lim f(x)$
\end{document}

And the output file is good. But when I activate the fontspec package, the compilation fails and shows the next error message:

! You can't use `\char' after \the.
\EU1\i ->\char
"0131\relax
l.27 $\limf(x)$
I'm forgetting what you said and using zero instead.
Missing character: There is no ^^@ in font [lmroman10-regular]:mapping=tex-text!
Missing character: There is no ^^S in font [lmroman10-regular]:mapping=tex-text!

What is the problem and how can I solve it?

Answer 1

I can reproduce the issue if I run the code with XeLaTeX on TeX Live 2014. If I also load unicode-math, the error disappears.

The problem is in how babel-spanish used to define the operator \lim in order to add the accent. In later releases of it, the issue has been removed.

A (not so) crude workaround if you can't upgrade:

\documentclass[letterpaper,reqno]{book}
\usepackage[spanish]{babel}
\usepackage{amssymb,amsmath,amsthm}
\usepackage{fontspec}

\makeatletter
\renewcommand\es@op@ac[1]{\textnormal{\'#1}}
\makeatother

\begin{document}

$\lim f(x)-\max g(x)+\min h(x)$

\end{document}