LuaLaTeX font fallback warning with luaotfload.add_fallback and luatexja

[BenjaminGalliot]

(Sorry to use English.)

I'm trying to use a virtual font combining several Unicode blocks, to have access to rarer characters very easily (without having to manually change the font).

The method I use in LuaLaTeX goes through luaotfload.add_fallback, but I get a warning (coming from luatexja).

I would like to know if this warning indicates a potential problem (particularly aesthetic, etc.), if it is possible to correct it, etc.

Here is the minimal working example (feel free to adapt your fonts!):

% !TEX program = lualatex

\documentclass{article}

\usepackage{ctex}

\directlua{
    luaotfload.add_fallback(
        "cjkfallback",
        {
            "TW-Kai-Ext-B-98_1:mode=harf;script=CJK;color=FF0000;",  % Red color to easily see fallbacks.
        }
    )
}

\setCJKfamilyfont{rm}[RawFeature={fallback=cjkfallback}]{TW-Kai-98_1}

\begin{document}

道𨕥

\end{document}

And the warning:

warning  (node filter): error: ...live/2023/texmf-dist/tex/luatex/luatexja/ltj-
jfmglue.lua:622: attempt to index a nil value (local 'm')

.
<argument> ...ype:D \tex_hskip:D \c_zero_dim \fi: \tex_par:D 
                                                  \hook_use:n {para/after}\@...

l.21 

Generally speaking, is there a better method with LuaLaTeX to create a virtual font combining several Unicode blocks?

[muzimuzhi]

Source code around line 622 in ltj-jfmglue.lua
https://github.com/luatexja/luatexja/blob/f6f0b5136dff5b5fac80e9b81cb59b2e917a44d2/src/ltj-jfmglue.lua#L615-L631

do
   -- ...

   function set_np_xspc_jachar(Nx, x)
      local m = ltjf_font_metric_table[getfont(x)] -- <<< m is defined here
      local c, c_glyph = (not getcomponents(x) and ltjs_orig_char_table[x]), getchar(x)
      if c and c~=c_glyph then set_attr(x, attr_jchar_code, c) end
      c = c or c_glyph
      local cls = slow_find_char_class(c, m, c_glyph)
      Nx.met, Nx.class, Nx.char = m, cls, c;
      local mc = m.char_type; Nx.char_type = mc    -- <<< line 622
      if cls~=0 then set_attr(x, attr_jchar_class, cls) end
      Nx.pre  = table_current_stack[PRE + c]  or 0
      Nx.post = table_current_stack[POST + c] or 0
      Nx.xspc = table_current_stack[XSP  + c] or 3
      Nx.kcat = table_current_stack[KCAT + c] or 0
      Nx.auto_kspc, Nx.auto_xspc
       = not has_attr(x, attr_autospc, 0), not has_attr(x, attr_autoxspc, 0)
      return m, mc, cls
   end

   -- ...
end