In the darcs darcs repository

darcs unpull  --from-match='author mornfall' --from-tag=2.0.2
darcs: bug in darcs!
Failure commuting patches in commute_by called by gpit!

We need to determine if this is a regression, first of all..

I tried with 2.0.2 and it fails too. Did not try with older versions.

The problem seems to be that darcs expects to have only one of from-match,
from-patch or from-tag flags. If we have both and from-tag goes after
from-match, in match_first_patchset function (called from get_last_patches) we
get the following:

- first_matcher is called, and returns matcher for from-match option.
- but first_matcher_is_tag - returns true because we have from-tag in options.
- so get_matching_tag is called with matcher for from-match and not for from-tag.

I hope you are able to understand my explanation :)

Unfortunately I do not know darcs internals enough to understand why exactly
failure happens. But if I replace first_matcher_tag call with False in
match_first_patchset darcs does not fail. And just ignores from-tag.

I see several ways to fix this:
- make first_matcher_is_tag work correctly when several from- options given
- complain and exit if more than one from- options are given

My understanding is that latter is the correct solution - several from- options
do not make sense.

In Darcs.Arguments match_from is defined as multiple choice option. AFAIK this
means that only one option from it should be specified. So maybe we should make
options parser to exit when there are several options from a single multiple
choice group given?

Regards,
  Dmitry

For what it's worth, I think I was trying to unpull patches by mornfall but only
up to the 2.0.2 tag. So one possible interpretation might be to think of tags as
somehow being special or wider matchers.  But maybe that's just my intuition and
not actually how things should work.

Rejecting multiple --from- matchers in the UI sounds like a plan, so I'm marking
this need-implementation.  Thanks for the analysis, Dmitry!