LaTeX build broken with LaTeX2e <2019-10-01> patch level 1 if X2 present in latex_elements['fontenc'] key (Cyrillic)c

[jfbu]

Describe the bug

PDF can not be build if conf.py contains

latex_elements = {
    'fontenc': r'\usepackage[LGR,X2,T1]{fontenc}',
}

which is documented method (search 'fontenc' key) to handle individual Greek (LGR) and Cyrillic (X2) letters since v2.0

To Reproduce

Insert the above in conf.py and issue make latexpdf for any project. For example with this index.rst.

Test
====

Σ (greek)
л (cyrillic)

Expected behavior

PDF builds succesfully and letters are ok in output.

Observed behavior

! LaTeX Error: Command \CYRYO unavailable in encoding T1.

happens during loading of sphinxcyrillic.sty file.

The console output contains the banner:

LaTeX2e <2019-10-01> patch level 1

I have not tested with initial LaTeX2e 2019-10-01> which got released on October 1st, and it would be a bit complicated to test only that version, so I did not try.

There is no problem with LaTeX from TeXLive 2018 nor, surely, with LaTeX earlier than the 2019-10-01 release.

Environment info

• OS: [e.g. Unix/Linux/Mac/Win/other with version]
• Python version: [e.g. 3.7.1]
• Sphinx version: release 2.0.0 or later
• Extra tools: Up-to-date TeXLive 2019 as of October 28, 2019

Additional context

The problem is only with Cyrillic, not with Greek.

This problem prevents building Sphinx own docs to PDF with current LaTeX from updated TeXLive 2019.

[jfbu]

However using T2A in place of X2 has no issue.

latex_elements = {
    'fontenc': r'\usepackage[LGR,T2A,T1]{fontenc}',
}

The sphinxcyrillic.sty has various code branches, and for X2 it uses a method which was posted by tex.stackexchange user egreg and used with his permission. The T2A branch uses another method which however needs to know the names used internally by LaTeX for each cyrillic glyph. This other method is not broken by recent LaTeX release.

[jfbu]

Fixed at 2ebdd0d