Provide a way to override particular settings from .fontspec files or at least ignore the files for particular font instances

[cfr42]

fontspec provides a highly usable interface enabling users to configure opentype fonts in sophisticated ways. However, the handling of .fontspec files undermines users ability to use this power.

Packaged .fontspec files are loaded by default and override settings specified explicitly by users at document level. Please correct me if I'm mistaken about this. (I keep thinking there must be a way, but have not found it.) This makes it unnecessarily (IMHO) difficult for people to override those settings.

It is always possible to write an empty or alternative .fontspec file, but this

• is not ideal for portability/reproducibility;
• does not allow using the .fontspec defaults for some instances but not others;
• means that changing a single default requires copying the whole file.

A relatively minor example: https://tex.stackexchange.com/q/757922/.

A related example:

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}
\setmathfont{NewCM10-Book.otf}[range={up}]
\begin{document}
a
\end{document}

fails to compile.

luaotfload | db : Reload initiated (formats: otf,ttf,ttc); reason: File not found: "--.otf".

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

For immediate help type H <return>.
 ...                                              
                                                  
l.508 \setmathfont{NewCM10-Book.otf}[range={up}]
                                                
? h

A font might not be found for many reasons.
 Check the spelling, where the font is installed etc. etc.

 When in doubt, ask someone for help!

Creating an empty NewCM10-Book.fontspec permits compilation, but obviously zaps all the default settings for all instances of the font.

I think fontspec should make it easy for users to override .fontspec defaults at the document level.

Something like

\setmainfont[Renderer=Node]{NewCM10-Book.otf}

should ensure the node renderer is used, regardless of any .fontspec file.

Similarly, something like

\setmainfont[fontspec=ignore]{NewCM10-Book.otf}

should load the font without reading any .fontspec at all. (Ideally, it should load a new instance, if necessary.)

I understand if the response to this is 'don't use fontspec if you don't like it'. That's fair enough and, if I start using LuaTeX, it is probably what I will do. But for many users, Unicode font configuration without fontspec is not a practical option.

[u-fischer]

This here compiles:

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}
\setmathfont{NewCM10-Book.otf}[IgnoreFontspecFile,range={up}]
\begin{document}
a
\end{document}

[wspr]

@cfr42 — this might not be said explicitly in the documentation but I nowadays believe it would be best for packaged .fontspec files not to use the filename of the font but rather an alias (e.g. "tgpagella.fontspec" instead of "texgyrepagella-regular.otf").

I don't think the intention of the .fontspec file feature was to override options set at the document level, I expect you've found an edge case due to the way that some options like Renderer are parsed early to ensure that later options are checked against the right mode. I'll leave this issue open to check that case.