[XeTeX] Bug with GFS fonts

[Will Robertson]

Hello,

I'm reporting this here because it's of interest to those here (and 
it's been reported as a bug in the past). There's a bug in the GFS 
fonts that causes superscripts to look wrong when using the xltxtra 
package. For example, a failing example would be

\documentclass{article}
\usepackage{xltxtra}
\setmainfont[Numbers=OldStyle]{GFS Didot}
\begin{document}
Hello\footnote{foo}
\end{document}

This can be isolated to a simple font declaration:

    {\fontspec[Numbers=OldStyle,VerticalPosition=Superior]{GFS Didot}123}

or (in plain XeTeX)

    {\font\1="GFS Didot:+onum,+sups"\1 123}

The OpenType specification for the sups feature says:

> Feature interaction: This feature may be used in combination with 
other substitution (GSUB) features, whose results it may override.

Therefore, the GFS fonts should allow this combination of font 
features. (The Adobe fonts, for example, behave correctly here.)

What's the easiest way for me to report this to the GFS people?

Many thanks,
Will