Problem with default bx series: Name and font substitution

[RuixiZhang42]

Description

The mtpro2 math package (and also the newtxmath math package, etc.) assumes that text font has a b series. Most traditional text font packages either provide the b series or make sure that font substitutions from b to bx and vice versa happen. This is not the case with fontspec.

Check/indicate

• Relevant for XeTeX
• Relevant for LuaTeX
• Issue tracker has been searched for similar issues?
• Originated from https://tex.stackexchange.com/a/479540

Minimal example demonstrating the issue

\documentclass{article}
\usepackage{fontspec}
\setmainfont{texgyretermes}[
  Extension=.otf,
  UprightFont=*-regular,
  ItalicFont=*-italic,
  BoldFont=*-bold,
  BoldItalicFont=*-bolditalic,
]
\usepackage[lite]{mtpro2}
%%% or
%\usepackage{newtxmath}
\begin{document}
\verb|      \mathnormal|: $012abc$\par
\verb|          \mathit|: $\mathit{012abc}$\par
\verb|          \mathbf|: $\mathbf{012abc}$\par
\verb|\boldmath+\mathit|: \boldmath$\mathit{012abc}$
\end{document}

Further details

One of the default font series set by fontspec is bx.

• AFAIK, bx is supposed to be “bold extended”, so the naming here is arguably wrong.
• Should fontspec make sure that font substitution between b and bx happen?

[RuixiZhang42]

Take t1ntxtlf.fd from the newtx collection for example, we find the following declarations:

\DeclareFontFamily{T1}{ntxtlf}{\ntx@spacing}
\DeclareFontShape{T1}{ntxtlf}{b}{sc}{<-> \ntx@scaled ntx-Bold-tlf-sc\ntx@lsfx-t1}{}
\DeclareFontShape{T1}{ntxtlf}{b}{n}{<-> \ntx@scaled ntx-Bold-tlf-t1}{}
...
\DeclareFontShape{T1}{ntxtlf}{bx}{sc}{<->ssub * ntxtlf/b/sc}{}
\DeclareFontShape{T1}{ntxtlf}{bx}{n}{<->ssub * ntxtlf/b/n}{}
...

i.e., newtxtext naturally only has bold. For compatibility reason, e.g., in case other packages do, say, \SetSymbolFont{operators}{bold}{\encodingdefault}{\rmdefault}{bx}{n}, then it will fall back from the non-existing bold extended to bold.

[RuixiZhang42]

A different point of view, as mentioned in your comment here, is to move the following code block:

  \DeclareSymbolFont{operators}\g_fontspec_encoding_tl\g_@@_mathrm_tl\mddefault\updefault
  \SetSymbolFont{operators}{normal}\g_fontspec_encoding_tl\g_@@_mathrm_tl\mddefault\updefault
  ...

to follow

  \bool_if:NT \g_@@_math_bool
   {
    \@@_info:n {setup-math}
    \fontspec_setup_maths:
   }
  % Add block here:

Thus, the \mathXX declarations are always executed regardless of the no-math option.

BTW, in the true branch of \tl_if_empty:NTF \g_@@_bfmathrm_tl, I think it is still necessary to set up

    \SetMathAlphabet\mathrm{bold}\g_fontspec_encoding_tl\g_@@_mathrm_tl\bfdefault\updefault % This line may be redundant
    \SetMathAlphabet\mathbf{bold}\g_fontspec_encoding_tl\g_@@_mathrm_tl\bfdefault\updefault

and in the false branch, the \mddefault choice is… debatable…

[RuixiZhang42]

It appears that only a few number of fonts (e.g., Computer Modern) provides the bx series.

• mtpro2 assumes that series b exists.
• newtxmath assumes that either b or sb exists. Its text package counterpart substitutes every bx series with b (in the .fd file).
• lmodern also substitutes every occurrence of bx with b.

An excerpt from the documentation:
https://github.com/wspr/fontspec/blob/c1fdf6d52c75017656c4d99ca5b4711353da125d/fontspec-doc-fontsel.tex#L486-L487

[RuixiZhang42]

@wspr Just noticed a large scale of bx -> b changes in the test files in 74c6917 and 1b3596c and 19fa825. Are these changes coming from the LaTeX2e kernel? I cannot find the code change in fontspec…

[wspr]

@RuixiZhang42 — yes, that's right, the new default in LaTeX2e will be b. Sorry for my silence on this issue... I will be adding bx↔︎b substitution some time soon...