LaTeX: user numfig_format is stripped of spaces and may cause build failure

[jfbu]

Procedure to reproduce the problem

index.rst:

====
test
====

Section
=======

.. figure:: foo.png

   is a figure

conf.py:

# -*- coding: utf-8 -*-

master_doc = 'index'
html_theme = 'classic'
numfig = True
numfig_format = {
    'figure': 'F %s F',
    'table':  'T %s T',
    'code-block': 'L %s L',
    'section': 'S %s S',
}

Do make latexpdf.

Error logs / results

! Undefined control sequence.
\fnum@figure ->\figurename \thefigureF 
                                       
l.85 \caption{is a figure}
                          \label{\detokenize{index:id1}}\end{figure}
? 

Expected results

PDF via LaTeX backends works: the stripped space before F causes \thefigureF non-existent macro to be used.

Remark1: by the way the default definition of \fnum@figure is \figurename\nobreakspace\thefigure. This explains why the default Fig. %s setting gives nevertheless a space in output.

Remark2: the HTML backend preserves the spaces.

Environment info

• OS: Mac OS
• Python version: 3.6.6
• Sphinx version: 1.8 branch at 7ff7dfc (1.8.4+)
• TeXLive 2018

Related: #5516, which migrates code that this issue, once fixed, will modify. Thus merging to master will need some care.

[jfbu]

Actually on closer look #5516 on master branch fixes this issue. But we will need some backport to 1.8 branch.