cat: should error when appending to input file redirected to stdin but doesn't #7165
Description
Environment: Ubuntu 20.04, uutil main branch (git commit d39dcad), GNU coreutils v9.6.3-3189c-dirty.
Steps to reproduce:
touch f && cat <f >>f
What happens now: uutils cat succeeds with no output.
What I expected to happen: GNU cat terminates with an error message:
cat: -: input file is output file
Notes: this is causing a failure in the GNU test file tests/cat/cat-self.sh.