/usr/share/fish/completions/tex.fish is in fish-common 2.2.0-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | set -l cmds -c etex -c tex -c elatex -c latex -c pdflatex -c pdfelatex -c pdftex -c pdfetex -c omega
__fish_complete_tex tex
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