allow nested .fontspec files to specify a font name

fontspec-doc-fontsel.tex

@@ -207,7 +207,7 @@ \subsection{By custom file name}
 This mechanism can be used to define custom names or aliases for your font collections.
 The syntax within this file follows from the \cs{defaultfontfeatures}, defined in more detail later but mirroring the standard \pkg{fontspec} font loading syntax.
 As an example, suppose we're defining a font family to be loaded with \verb|\setmainfont{My Charis}|.
-The corresponding \texttt{MyCharis.fontspec} file would containing, say,
+The corresponding \texttt{MyCharis.fontspec} file would contain, say,
 \begin{Verbatim}
   \defaultfontfeatures[My Charis]
     {
@@ -227,8 +227,28 @@ \subsection{By custom file name}
   \defaultfontfeatures[CharisSILR]{Color=blue}
   \defaultfontfeatures[CharisSILB]{Color=red}
 \end{Verbatim}
-Such configuration lines could be stored either inline inside \texttt{My Charis.fontspec} or within their own \texttt{.fontspec} files; in this way, \pkg{fontspec} is designed to handle `nested' configuration options.
+Such configuration lines could be stored either inline inside \texttt{My Charis.fontspec}
+or within their own \texttt{.fontspec} files; in this way, \pkg{fontspec} is designed to
+handle `nested' configuration options.
 
+Where \cs{defaultfontfeatures} is being used to specify font faces by a custom name,
+the \feat{Font} feature is used to set the filename of the font face. For example:
+\begin{Verbatim}
+  \defaultfontfeatures[charis]
+    {
+      UprightFont = charis-regular,
+      % <other desired options for all font faces in the family>
+    }
+
+  \defaultfontfeatures[charis-regular]
+    {
+      Font = CharisSILR
+      % <other desired options just for the `upright' font>
+    }
+\end{Verbatim}
+The \pkg{fontspec} interface here is designed to be flexible to accomodate a variety
+of use cases; there is more than one way to achieve the same outcome when font faces are
+collected together into a larger font family.
 
 
 \subsection{Querying whether a font `exists'}

fontspec-internal.dtx

@@ -676,10 +676,19 @@
     \@@_load_external_fontoptions:Nn \l_fontspec_fontname_tl {#1}
     \prop_get:NVNF \g_@@_fontopts_prop \l_fontspec_fontname_tl \l_@@_fontopts_clist
      { \clist_clear:N \l_@@_fontopts_clist }
+    \keys_set_groups:nnV {fontspec} {getfontname} \l_@@_fontopts_clist
     \@@_primitive_font_set:Nnn \l_fontspec_font { \@@_construct_font_call:nn {\l_fontspec_fontname_tl} {} } {\f@size pt}
     \@@_primitive_font_if_null:NT \l_fontspec_font { \@@_error:nx {font-not-found} {#1} }
  }
 %    \end{macrocode}
+%
+%    \begin{macrocode}
+\keys_define:nn {fontspec}
+  {
+    Font .tl_set:N = \l_fontspec_fontname_tl ,
+    Font .groups:n = {getfontname} ,
+  }
+%    \end{macrocode}
 % \end{macro}
 %
 % \begin{macro}{\@@_declare_shape:nnnn}

fontspec-keyval.dtx

@@ -171,10 +171,6 @@
   {
     \fontspec_complete_fontname:Nn \l_@@_fontname_up_tl {#1}
   }
-\@@_keys_define_code:nnn {fontspec-preparse-external} {FontName}
-  {
-    \fontspec_complete_fontname:Nn \l_@@_fontname_up_tl {#1}
-  }
 %    \end{macrocode}
 %
 % \paragraph{Italic and slanted}