[GHC] #10931: layers-0.1 does not compile with ghc-7.10 (likely a regression from ghc-7.8)

[GHC]

#10931: layers-0.1 does not compile with ghc-7.10 (likely a regression from
ghc-7.8)
-------------------------------------+-------------------------------------
        Reporter:  slyfox            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * cc: goldfire (added)


Comment:

 Thanks for a small test case.  I've added a test so if this goes wrong
 again we'll know.

 I have not investigated in detail.  Would someone like to git-bisect to
 find when it got fixed?  I have a horrid feeling that it'll just be part
 of some fairly substantial change to the type constraint solver, which we
 won't want to merge en-masse to 7.10.3.

 As things stand, it's a bug in 7.10 that we don't know how to fix.  I'm
 sure we could find out, but it'd take precious cycles.

 How bad is it if this library doesn't work?  I imagine this is relatively
 rare else it'd show up more often.

 Richard; you might be interested.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10931#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler