Arch Linux

requires help2man and rst2man during build(), and the Makefile does not exit with an error if they're not available.

This is due to using pipelines without pipefail, which is not guaranteed to exist anyway unless you actually use `SHELL = /bin/bash` in your Makefile.

I've installed both via the help2man and python-docutils packages (and verified that both commands were available), but after a `sudo pacman --sync diffoscope` the symptoms persist. Ditto after `sudo pacman --remove diffoscope && sudo pacman --sync diffoscope`.

I, um, don't quite understand why you think that would help? My comment was a reference to the package maintainer, regarding the necessary steps for making the source code properly create the man page. Are you downloading the diffoscope source code and trying to repackage the software?

tl;dr
If you don't understand what something is referring to, blindly installing software won't help. If your intent was to fork the repository package and rebuild it with local modifications so you will have the manpage while waiting for this bug to be fixed, then you have forgotten the final step: https://wiki.archlinux.org/index.php/Arch_Build_System#Why_would_I_want_to_use_ABS.3F

I thought "requires help2man and rst2man during build(), and the Makefile does not exit with an error if they're not available" meant that if I had those available they would be used by the current build script, and that the rest was simply instructions to help the package maintainer modify the build script to fail if they are not available. No problem, I'll keep my hands off.

Yes, the build script is run by the package maintainer. ;)