I have a local copy of the darcs darcs repository in which I can systematically
reproduce this error by pulling in the current unstable repository.

I have created a tarball for future investigation.

The procedure in
  http://wiki.darcs.net/DarcsWiki/Forensics
may come in handy (or not)

I can reproduce this by applying the attached bundle to the attached
ctx (ctx)

Attachments

• ctx
• move-patch_filename-out-of-uglyfilename.dpatch

The attached boom.dpatch is smaller and also makes darcs blow up on the repo
mentioned in the context file I previously attached

Attachments

• boom.dpatch

The code at the point of pattern failure has a bunch of commented out debug 
statements.  I enabled then and ran it again.  The snippet of the trace right 
before the crash looks like this:

Conflictor a1 n1 p1 is
conflictor [
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
]
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9] smart_diff unsafeDiff
and Conflictor a2 n2 p2 is
conflictor {{
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9] smart_diff unsafeDiff
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
}} []
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] takePS B.take
where n2'nons is
and others are
These came from
{{
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
}}
n1'n2'nons
{{
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
}}
from n1' :> n2'
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
:>
p2
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] takePS B.take
p2 fixed
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] takePS B.take
n1'
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] PackedString B.ByteString
p2
|:
replace ./src/Darcs/Diff.lhs [A-Za-z_0-9\-\.] takePS B.take
darcs: src/Darcs/Patch/Real.hs:(482,21)-(500,99): Irrefutable pattern failed for 
pattern Data.
Maybe.Just a2'o

Marking as a release target for darcs 2.3 (please override as needed, Thorkil)

I think it would still be useful to attempt to create a minimal example out of
this (since we have something that's reliably reproducible)

We are not likely to get this fixed before 2.5. I bump it to 2.6.

This was opened in 2008 and has been bumped from release to release since 
then. We aren't like to get it to for 2.8, either. Bumping to 2.10.