All great comments, thank you! I feel the need to reply, even though there isn't a real need to, but here goes:

1. True; but in this case we're talking about bash specifically, so I prefer to use cleaner syntax whenever this is available. I find ${var+x} a bit too hacky ...

2. Huh, I was under the impression that bespoke commands overrode builtins. Good to know. What the hell is even the reason to have a 'builtin' keyword then ... just for "alias test=/usr/bin/test" style scenarios??

3. True about $#. Not sure why I didn't think of that, d'oh.

4. Thanks re Shellcheck. I do have it in mind, and was planning to use it prior to releasing a v1; but process_optargs came out of a larger project which is still in v0 (https://git.sr.ht/~tpapastylianou/bashtickets), and I only forked it into its own thing because someone asked me about it on HN, so I lost track. Thanks for the reminder. Having said that, I can't see why the error you describe would occur in this instance. Would you mind explaining? Why would the presence of a single-letter filename have anything to do with the piped output to tr?

5. Good point about printf (or about habitually passing "--" as an argument to things, I guess).

Thanks again!