id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
4966,Type checking regression,igloo,simonpj,"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.
",bug,closed,highest,7.0.2,Compiler (Type checker),7.0.1,fixed,,,Unknown/Multiple,Unknown/Multiple,None/Unknown,,deriving/should_compile/T4966,,,