[GHC] #4360: compiler crash building hashed-storage 0.5.3

[GHC]

#4360: compiler crash building hashed-storage 0.5.3
---------------------------------+------------------------------------------
    Reporter:  ganesh            |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.13              |    Keywords:              
    Testcase:                    |   Blockedby:              
          Os:  Unknown/Multiple  |    Blocking:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.0.0.20100924 for x86_64-unknown-linux):
         Expected one of wanted constraints (BUG)
 }}}

 This happens with {{{cabal install hashed-storage-0.5.3}}}, but that's a
 bit of a pain to reproduce as it depends on dataenc which needs a
 dependency bump. I cut it down to the attached file which just depends on
 mtl.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4360>
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