Font name stored in a string variable is not correctly parsed

[eg9]

Description

If a font name with extension is stored in a string variable, neither LuaLaTeX nor XeLaTeX treat it as expected: with LuaLaTeX a warning is issued, but the font is eventually loaded; with XeLaTeX the font is not found.

The problem is that the allowed extensions are stored in a clist as tokens with category code 11 (for the letters), so no match is detected.

Check/indicate

• Relevant for XeTeX
• Relevant for LuaTeX
• Issue tracker has been searched for similar issues?
• https://tex.stackexchange.com/questions/729423/why-filenames-defined-through-expl3-strings-dont-work-with-setmainfont

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{fontspec}

\ExplSyntaxOn
\str_const:Nn \c_my_font_name_str {Erewhon-Regular.otf}
\setmainfont{\c_my_font_name_str}
\ExplSyntaxOff

\begin{document}

Text

\end{document}

Further details

Relevant part of the console output with LuaLaTeX

luaotfload | db : Reload initiated (formats: otf,ttf,ttc); reason: Font "Erewhon-Regular.otf" not found.
(./dbfont.aux) (/usr/local/texlive/2024/texmf-dist/tex/latex/base/ts1cmr.fd)
[1{/usr/local/texlive/2024/texmf-var/fonts/map/pdftex/updmap/pdftex.map}]
(./dbfont.aux))
 406 words of node memory still in use:
   3 hlist, 1 vlist, 1 rule, 2 glue, 3 kern, 1 glyph, 4 attribute, 48 glue_spec
, 4 attribute_list, 1 write nodes
   avail lists: 1:1,2:22,3:4,4:1,5:23,6:2,7:36,9:18
</Library/Fonts/erewhon/Erewhon-Regular.otf>

Relevant part of the console output with XeLaTeX

mktextfm: Running mf-nowin -progname=mf \mode:=ljfour; mag:=1; ; nonstopmode; input Erewhon-Regular
This is METAFONT, Version 2.71828182 (TeX Live 2024) (preloaded base=mf)


kpathsea: Running mktexmf Erewhon-Regular
! I can't find file `Erewhon-Regular'.
<*> ...ag:=1; ; nonstopmode; input Erewhon-Regular

Please type another input file name
! Emergency stop.
<*> ...ag:=1; ; nonstopmode; input Erewhon-Regular

Transcript written on mfput.log.
grep: Erewhon-Regular.log: No such file or directory
mktextfm: `mf-nowin -progname=mf \mode:=ljfour; mag:=1; ; nonstopmode; input Erewhon-Regular' failed to make Erewhon-Regular.tfm.
kpathsea: Appending font creation commands to missfont.log.


! Package fontspec Error:
(fontspec)                The font "Erewhon-Regular.otf" cannot be found;
(fontspec)                this may be but usually is not a fontspec bug.
(fontspec)                Either there is a typo in the font name/file, the
(fontspec)                font is not installed (correctly), or there is a
(fontspec)                bug in the underlying font loading engine
(fontspec)                (XeTeX/luaotfload).

Hackish workaround

\cs_set:Nn \__fontspec_main_DeclareFontExtensions:n
  {
    \clist_set:Ne \l__fontspec_extensions_clist { #1 , \tl_to_str:n {#1} }
  }
\__fontspec_main_DeclareFontExtensions:n {.otf,.ttf,.OTF,.TTF,.ttc,.TTC,.dfont}

This stores the extensions both with normal catcodes and with catcode 12 for every character, so the match is found anyway.

Long term proposal

Do the entire business with strings.