Fix #9305 LaTeX: backslash in sphinxupquote error with Japanese

[jfbu]

Relates

• Fix sphinxupquote and textbackslash: Improper discretionary list. #9305

[jfbu]

I should perhaps use a proper scratch box but the contents of \sphinxupquote should be a text string hence it seems legit to use say \box8 as here (as this is enclosed in braces).

The \textbackslash is a complex command which checks the current font encoding and triggers a fall-back font encoding (OMS, math character) for example if character not available. So the former method with \let did not have expected effect and the \textbackslash was trying to redefine itself in a \discretionary context where only boxes are allowed.

[jfbu]

(I mentioned OMS from my pure latex testing, but in Sphinx context this will be T1 encoding)

[jfbu]

I used this test file from a former issue with backslash and line-wrapping in literal environments.

Welcome to FOO's documentation!
===============================

:file:`a\\b`

:file:`\\C\\a{}a{}a{}\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa\\aaa`

::

   \aaa

``\aaa``

:file:`\\C\\aaa`


::

   \a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\a\

AAA

.. parsed-literal::

   \\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\a\\

AAA

.. raw:: latex

         \par\hrule\par

:file:`a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\a\\\\aa`

I tested with language='ja' and it works.

[jfbu]

Thanks for review, I merge now.