[GHC] #7296: ghc-7 assumes incoherent instances without requiring language `IncoherentInstances`

[GHC]

#7296: ghc-7 assumes incoherent instances without requiring language
`IncoherentInstances`
-----------------------------------------+----------------------------------
 Reporter:  maeder                       |          Owner:                  
     Type:  bug                          |         Status:  new             
 Priority:  normal                       |      Component:  Compiler        
  Version:  7.6.1                        |       Keywords:                  
       Os:  Unknown/Multiple             |   Architecture:  Unknown/Multiple
  Failure:  GHC accepts invalid program  |       Testcase:                  
Blockedby:                               |       Blocking:                  
  Related:                               |  
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 the attached examples works with ghc-7 and returns

 {{{
     *Main> a
     [Spec1 Spec2]
     *Main> b
     []
 }}}

 (One may wish that b also returned [Spec1 Spec2])

 ghc-6 complains with

 {{{
 Splittable.hs:16:36:
     Overlapping instances for Test a Spec2
       arising from a use of `test' at Splittable.hs:16:36-43
     Matching instances:
       instance [overlap ok] Test a Spec2
         -- Defined at Splittable.hs:20:13-24
       instance [overlap ok] Test Bool Spec2
         -- Defined at Splittable.hs:25:13-27
     (The choice depends on the instantiation of `a'
      To pick the first instance above, use -XIncoherentInstances
      when compiling the other instance declarations)
     In the second argument of `($)', namely `test a b'
     In the expression: map Spec1 $ test a b
     In the definition of `test':
         test a (Spec1 b) = map Spec1 $ test a b
 Failed, modules loaded: none.
 }}}

 After adding `IncoherentInstances` the file also goes through ghc-6 with
 the same results.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7296>
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