In this way, one can implement _comp_xfunc_CMD_compgen_NAME using _comp_comgen and let the caller decide whether to replace or append. It replaces by default the same as the other _comp_compgen_NAME. If one wants to append the new completions, one can call it using _comp_compgen -a CMD/NAME args....

Sounds good to me in principle, although the CMD/NAME syntax feels a bit magical. How about moving the xfunc cmd to [-x CMD]?

2 with -i looks best to me of the options so far. Given green field, -c would look even better (-x ~ xfunc, -c ~ cmd) but it's taken for cur already. Perhaps -m ~ cMd could be worth considering, but is quite far fetched.

In a sense I like 3 too, it would kind of make it clearer that it's really for the same cmd namespace and not to be used from anywhere else, but... dunno. Calling from somewhere else breaks immediately because there's no sourcing mechanism with it, so not that much of a problem.

Thoughts on the above 2 vs 3 consideration? In particular, I'm wondering if the redundant CMD with -i is worth the symmetry.

I realized that omitting CMD can be a problem when the completion is loaded through a symbolic link, in which case BASH_SOURCE will tell the path of the symbolic link. For this reason (in addition to the symmetry), I currently favor option 2.

We might still call readlink against "${BASH_SOURCE[1]}", but the implementation would become complicated and it requires spawning a process.

• First, readlink is not standardized in POSIX. For this problem, we can extract the link target from the result of ls -l <file> instead of relying on readlink.
• Even if readlink is available, we might need to resolve recursive links since readlink -f or realpath is not universally available. That said, we may not need to care about recursive links for completions/*.
• To reduce the spawning cost, we might first check [[ -h ${BASH_SOURCE[1]} ]] before calling ls -l and only call ls -l when it is a symbolic link. For the second call of the same internal generator, we might create a delegating function _comp_cmd_LINK__compgen_NAME() { _comp_cmd_CMD__compgen_NAME "$@"; } on the fly.
• Also, BASH_SOURCE[1] can contain a relative path to the file from the working directory when the file was loaded. We can make sure to use the absolute path in __load_completion.

I agree, thanks for taking the time to explain 👍