Unhelpful IncoherentInstances suggestion with FunctionalDependencies

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 (closed) and #7171 (closed) might be related.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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