Can not build Sphinx own PDF docs since 4.1.0

[jfbu]

Describe the bug

Sphinx own PDF document build fails:

LaTeX Warning: Command \TeX invalid in math mode on input line 22151.

! You can't use `\spacefactor' in math mode.
\@->\spacefactor 
                 \@m {}
l.22151 ... that the system has a working \(\LaTeX
                                                  \) installation,
? 

https://github.com/sphinx-doc/sphinx/pull/9355/files#r672173343

How to Reproduce

build Sphinx own docs

Sphinx version

4.1.0, 4.1.1

[astrojuanlu]

I see this in the RTD logs:

LaTeX Warning: Command \LaTeX invalid in math mode on input line 22894.


LaTeX Warning: Command \TeX invalid in math mode on input line 22894.

! You can't use `\spacefactor' in math mode.
\@->\spacefactor 
                 \@m {}
l.22894 ... that the system has a working \(\LaTeX
                                                  \) installation,
...
Latexmk: Log file says output to 'sphinx.pdf'
Collected error summary (may duplicate other messages):
  pdflatex: Command for 'pdflatex' gave return code 256
Latexmk: Errors, in force_mode: so I tried finishing targets

however, the job didn't fail. I wonder if we should consider this a bug on the RTD side @humitos

[astrojuanlu]

URL https://readthedocs.org/projects/sphinx/builds/14214434/

[mgeier]

@astrojuanlu

however, the job didn't fail. I wonder if we should consider this a bug on the RTD side

See also readthedocs/readthedocs.org#7884.

[jfbu]

however, the job didn't fail. I wonder if we should consider this a bug on the RTD side @humitos

In this specific case there is (strangely) this line in latexmk console output:

Latexmk: Summary of warnings from last run of *latex:
  =====Latex reported missing or unavailable character(s).

Generally speaking, such a warning is indication the PDF is not 100% correct. But how to fix this will generally be not obvious.

And in this specific case, this warning is rather misleading. We find a particularly strange message in the latex log:

Missing character: There is no è in font cmex10!

which is not of much help and is another symptom of $\LaTeX$. Besides the PDF output is not that broken in this specific case.

But generally speaking a

  =====Latex reported missing or unavailable character(s).

should be cause for alarm. Not here, but in general.