[GHC] #7875: Unhelpful IncoherentInstances suggestion with FunctionalDependencies

[GHC]

#7875: Unhelpful IncoherentInstances suggestion with FunctionalDependencies
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Reporter:  dreixel           |          Owner:                  
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.7               |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
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 Consider the following program (simplified from http://hpaste.org/86928):
 {{{
 {-# LANGUAGE
     FlexibleContexts
   , FlexibleInstances
   , FunctionalDependencies
   , MultiParamTypeClasses
   , KindSignatures
   , UndecidableInstances #-}

 class Het a b | a -> b where
   het :: m (f c) -> a -> m b

 class GHet (a :: * -> *) (b :: * -> *) | a -> b
 instance            GHet (K a) (K [a])
 instance Het a b => GHet (K a) (K b)


 data A a   = A (A a)
 data K x a = K x

 instance Het (A a) (A [a]) where het = het1

 het1 :: (GHet (K a) (K b)) => m (f c) -> a -> m b
 het1 = undefined
 }}}

 In HEAD, it gives rise to the following error:
 {{{
     Overlapping instances for GHet (K (A a)) (K (A [a]))
       arising from a use of ‛het1’
     Matching instances:
       instance Het a b => GHet (K a) (K b) -- Defined at Bug.hs:14:10
     There exists a (perhaps superclass) match:
     (The choice depends on the instantiation of ‛a’
      To pick the first instance above, use -XIncoherentInstances
      when compiling the other instance declarations)
     In the expression: het1
     In an equation for ‛het’: het = het1
     In the instance declaration for ‛Het (A a) (A [a])’
 }}}

 It's already strange that it says multiple instances match, but it only
 lists one. Furthermore, enabling `-XIncoherentInstances`, as suggested,
 does not change the error message.

 #7150 and #7171 might be related.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7875>
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