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From: Stephane Chazelas <stephane.chazelas@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: $? is -1
Date: Fri, 5 Jul 2019 19:03:16 +0100
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To: Chet Ramey <chet.ramey@case.edu>
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2019-07-05 09:03:06 -0400, Chet Ramey:
[...]
> > 	Second, when ran in a subshell, the same exit status gets mapped to 255.
> > 	While logical, as -1 gets mapped to 255, it seems inconsistent.
> > 	( from the manual: "The return status is the exit status of list." )
> 
> It's the difference between passing a status around the shell and passing
> it through the waitpid() interface, which takes just eight bits.
[...]

Note though:

$ bash -c 'f() { return "$1"; }; f -1; echo "$?"'
255

It gets mapped to 255 even though there's no waitpid().

bash also takes upon itself to truncate the number passed to
exit(1) before passing it to _exit(2):

$ strace -fe exit_group bash -c 'exit -1'
exit_group(255)                         = ?

There's a lot of variation between shells (and other utilities)
on that front. See also:

https://unix.stackexchange.com/questions/418784/what-is-the-min-and-max-values-of-exit-codes-in-linux/418802#418802
https://unix.stackexchange.com/questions/99112/default-exit-code-when-process-is-terminated/99134#99134

-- 
Stephane