No warning is issued against `[ ! -a p ]` and `test ! -a p`

[e-kwsm]

For bugs

• Rule Id (if any, e.g. SC1000): SC2166?
• My shellcheck version (shellcheck --version or 'online'): online
• I tried on shellcheck.net and verified that this is still a problem on the latest commit
• It's not reproducible on shellcheck.net, but I think that's because it's an OS, configuration or encoding issue

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Here's a snippet or screenshot that shows the problem:

#!/bin/bash
# or
#!/bin/ksh
[ ! -a /bin/sh ]
echo $?

Here's what shellcheck currently says:

nothing

Here's what I wanted or expected to see:

A warning is issued against [ ! -a p ] since it is not well-defined.
Actually, bash 5.2.37 and ksh 2020.0.0 give different results: 0 and 1, respectively. It seems that bash performs

[ ! ] && [ /bin/sh ]
# or
[ -n '!' ] && [ -n /bin/sh ]

whereas ksh does

! [ -a /bin/sh ]
# or
! [ -e /bin/sh ]

[e-kwsm]

set -B
[ ! -o braceexpand ]
echo $?  # bash: 0, ksh: 1
[[ ! -o braceexpand ]]
echo $?  # bash: 1, ksh: 1

# shellcheck should suggest
! [ -o braceexpand ]

[koalaman]

ShellCheck now generally recommends -e over unary -a, warns about [ -o opt ] not being POSIX when applicable, and about the pitfall of [ ! -o braceexpand ]. Very surprising behavior and a great check, thanks!