[XeTeX] Required feature not applied

[Ralf Stubner]

Hi,

when trying to debug some problems with the Junicode font I found that
it looks as if under certain circumstances XeTeX does not apply the
'required feature' specified in a lookup. In particular the [Tt]horn in
the following example is not replaced by its 'nordic' variant even
though this is a required feature for latn{ISL} in that font:

%%%%%%%%%%%%%%%%%%%%%%
\font\a="Junicode-Regular"\a
þ Þ fi
\font\b="Junicode-Regular:language=ISL " \b
þ Þ fi

\bye
%%%%%%%%%%%%%%%%%%%%%%

One can see that latn{ISL} is applied though, as the 'fi'-ligature is
lost, since the required feature is actually the only feature in that
lookup: 

,----[ ttx output ]
|           <LangSysRecord index="0">
|             <LangSysTag value="ISL "/>
|             <LangSys>
|               <ReqFeatureIndex value="0"/>
|               <!-- FeatureCount=0 -->
|             </LangSys>
|           </LangSysRecord>
`----

That the latn{dflt} features are not replicated in latn{ISL} is a bug in
the font, but still, the required feature should be applied by XeTeX.
Interestingly if I add some other feature to this lookup, XeTeX does
apply the required feature.

What's wrong here?

cheerio
ralf