The following crashes bash:
bash-5.0$ set -u
bash-5.0$ echo ${!,}
bash: $!: unbound variable
bash: !: unbound variable
bash-5.0$ echo ${!,}
bash: $!: unbound variable
Testing if $! is set also doesn't seem to be possible:
bash-5.0$ echo ${!-}
bash: $!: unbound variable
