[Tex/LaTex] File l3backend-pdfmode.def not found when loading expl3

expl3packages

I updated my TeXlive distribution a couple times in the past week, and now I get this error when I include package expl3 (or any other package that includes it, such as fontspec).
Before the updates it was working fine.
Am I missing any package that should be installed? I tried searching for l3backend-pdfmode.def, but I found no results.

\documentclass{article}
\usepackage{expl3}
\begin{document}
Hi
\end{document}
λ lualatex a
This is LuaTeX, Version 1.10.0 (TeX Live 2019) 
 restricted system commands enabled.
(./a.tex
LaTeX2e <2018-12-01>

luaotfload | main : initialization completed in 0.042 seconds
(/opt/texlive/2019/texmf-dist/tex/latex/base/article.cls
Document Class: article 2018/09/03 v1.4i Standard LaTeX document class
(/opt/texlive/2019/texmf-dist/tex/latex/base/size10.clo))
(/opt/texlive/2019/texmf-dist/tex/latex/l3kernel/expl3.sty
(/opt/texlive/2019/texmf-dist/tex/latex/l3kernel/expl3-code.tex)

! LaTeX Error: File `l3backend-pdfmode.def' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: def)

Enter file name: 
! Emergency stop.
<read *> 

l.282   }

 379 words of node memory still in use:
   2 hlist, 1 rule, 1 dir, 3 kern, 1 glyph, 4 attribute, 48 glue_spec, 4 attrib
ute_list, 1 write nodes
   avail lists: 2:9,3:3,4:1,5:2,7:2,9:2
!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on a.log.

Best Answer

Since the July 1st, 2019 release of the LaTeX3 kernel, the driver-specific code contained in the l3backend module was moved to a dedicated package in CTAN, and now l3backend is a dependency of l3kernel.

The description of l3backend on CTAN says:

The func­tions here are de­fined dif­fer­ently de­pend­ing on the en­gine in use. As such, these are dis­tributed sep­a­rately from l3k­er­nel to al­low this code to be up­dated on an in­de­pen­dent sched­ule.

So if you updated l3kernel only without installing l3backend as well you'll get an error like this:

($TEXMFROOT/texmf-dist/tex/latex/l3kernel/expl3-code.tex)

! LaTeX Error: File `l3backend-<driver-name>.def' not found.

Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: def)

Enter file name: 

and the solution is to install the missing l3backend package.

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