Type checking regression

This module:

{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}

module HTk.Toolkit.TreeList (getObjectFromTreeList) where

class Eq c => CItem c

data StateEntry a = StateEntry (TreeListObject a)
                               a -- Comment this 'a' out and it type checks
    deriving Eq

getObjectFromTreeList :: CItem a => [StateEntry a] -> Bool
getObjectFromTreeList state = (head state == head state)

data CItem a => TreeListObject a

instance CItem a => Eq (TreeListObject a)

class GUIObject w where
  toGUIObject     :: w -> GUIOBJECT

instance GUIObject w => Eq w where
  w1 == w2 = toGUIObject w1 == toGUIObject w2

data GUIOBJECT

instance Eq GUIOBJECT where
  (==) = undefined
  (/=) = undefined

is accepted by 7.0.1, but not by 7.0-branch or HEAD:

Q.hs:16:43:
    Could not deduce (GUIObject a) arising from a use of `=='
    from the context (CItem a)
      bound by the type signature for
                 getObjectFromTreeList :: CItem a => [StateEntry a] -> Bool
      at Q.hs:16:1-56
    Possible fix:
      add (GUIObject a) to the context of
        the type signature for
          getObjectFromTreeList :: CItem a => [StateEntry a] -> Bool
    In the expression: (head state == head state)
    In an equation for `getObjectFromTreeList':
        getObjectFromTreeList state = (head state == head state)

If you comment out the a then it is accepted by all 3.

Trac metadata

Trac field	Value
Version	7.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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