[GHC] #7150: unjustified overlapping instances error

[GHC]

#7150: unjustified overlapping instances error
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 Reporter:  maeder                     |          Owner:          
     Type:  bug                        |         Status:  new     
 Priority:  normal                     |      Component:  Compiler
  Version:  7.6.1-rc1                  |       Keywords:          
       Os:  Solaris                    |   Architecture:  x86     
  Failure:  GHC rejects valid program  |       Testcase:          
Blockedby:                             |       Blocking:          
  Related:                             |  
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 When compiling hets (http://trac.informatik.uni-
 bremen.de:8080/hets/wiki/HetsForDevelopers) using ghc-7.6.0.20120810 I get
 the following error

 {{{
 [489 of 664] Compiling Comorphisms.HasCASL2IsabelleHOL (
 Comorphisms/HasCASL2IsabelleHOL.hs, Comorphisms/HasCASL2IsabelleHOL.o )

 Comorphisms/HasCASL2IsabelleHOL.hs:61:44:
     Overlapping instances for Category Env Morphism
       arising from a use of `sourceSublogic'
     Matching instances:
       instance [incoherent] Category Env Morphism
         -- Defined at HasCASL/Logic_HasCASL.hs:74:10
     There exists a (perhaps superclass) match:
     (The choice depends on the instantiation of `'
      To pick the first instance above, use -XIncoherentInstances
      when compiling the other instance declarations)
     In the second argument of `isSubElem', namely `sourceSublogic cid'
     In the expression: sl `isSubElem` sourceSublogic cid
     In the expression:
       if sl `isSubElem` sourceSublogic cid then Just () else Nothing
 }}}

 Note, that only a single instance is listed and that the choice is empty.
 Adding -XIncoherentInstances does also not help to get it compiled.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7150>
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