\input and \file_if_exist:nTF search diffrent paths in texlive-lualatex.

[Karl Berry]

    If I have child.tex in a
    subfolder, and then call pdflatex --output-directory on this file:

    \documentclass{article}

    \begin{document}
    \input{child}

    \input{child.tex}
    \end{document}

If you're reporting that the .tex extension is not automatically added
for --output-directory searches, that's intentional and will not change.
(I'm not sure if you're reporting that, or something about subdirs.)

Long ago I wrote about this in the web2c manual.
The last paragraph in this section:
  https://tug.org/texinfohtml/web2c.html#Output-file-location

says:
    TEXMFOUTPUT is also checked for input files, as TeX often generates
    files that need to be subsequently read; for input, no suffixes
    (such as tex') are added by default and no exhaustive path searching
    is done, the input name is simply checked as given.

If/Since Christian decided to change that, that's fine for him, but I
don't think it's desirable to do anything but check the name as given in
TEXMFOUTPUT.  That search is special and we wouldn't want to find the
wrong thing. --thanks, karl.