Ceph

This is at least helped by commit:7e1c73538f44ad83db476ffa15d5121470b5154e in wip-6332 (pull request: https://github.com/ceph/ceph/pull/606).

Not sure what you'd want to happen here; the "semantic reason for the failure of the command" is a bit of a stretch. We could maybe print the help for the failing command if there's only one that could have matched?....

Yeah, that would probably be good. It's clearly gotten to the point that it knows what command it's looking at, but while as a developer I eventually figured out what was going on, "saw 0 of pool2(<poolname>), expected 1" is meaningless without the source.

Yeah, it's not the best message. Maybe we should special case the 'saw 0 expected 1' case to something more intelligible, like "must supply <pool2>" (since it's by far the
most common case)

With both those suggestions implemented, the output looks like this:

$ ./ceph osd pool delete data
*** DEVELOPER MODE: setting PATH, PYTHONPATH and LD_LIBRARY_PATH ***
Invalid command:  missing required parameter pool2(<poolname>)
osd pool delete <poolname> <poolname> --yes-i-really-really-mean-it: delete pool (say pool twice, add --yes-i-really-really-mean-it)
Error EINVAL: invalid command

Does that seem like a reasonable result?

well, I've created a pull request; if you want to review, now's your chance