Problem with default `bx` series: Name and font substitution

RuixiZhang42 commented 5 years ago

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

[x] Relevant for XeTeX
[x] Relevant for LuaTeX
[x] Issue tracker has been searched for similar issues?
[x] 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 commented 5 years ago

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 commented 5 years ago

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 commented 5 years ago

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 commented 4 years ago

@wspr Just noticed a large scale of bx -> b changes in the test files in https://github.com/wspr/fontspec/commit/74c6917a13faa3aef57a68820ebaa222d51e4b42 and https://github.com/wspr/fontspec/commit/1b3596ce63db3ed73592144ffec5e42d17000c59 and https://github.com/wspr/fontspec/commit/19fa825a56c144ad478cc9f42bf2c0ff80e7fd4e. Are these changes coming from the LaTeX2e kernel? I cannot find the code change in fontspec…

wspr commented 4 years ago

@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...

latex3 / fontspec