Example with multi-parameter type classes and dependent types that ghc-6.2.1 compiles but ghc-6-2-branch doesn't

[George Russell]

With ghc-6.2.1

# ghc -c Folders.hs -fglasgow-exts -fallow-undecidable-instances

(compiles with warnings)

With ghc-6-2-branch, downloaded from the read-only repository yesterday:

# /local/home/ger/fptools.install/bin/ghc -c Folders.hs -fglasgow-exts -fallow-undecidable-instances

Folders.hs:31:
     Could not deduce (HMV x graph node)
	from the context (HMV NodeArcsHidden graph node)
       arising from use of `modd' at Folders.hs:31
     Probable fix:
	Add (HMV x graph node) to the type signature(s) for `gn'
     In a lambda abstraction: \ graph node -> modd arcsHidden graph node
     In a lambda abstraction:
	\ arcsHidden -> (\ graph node -> modd arcsHidden graph node)
     In the first argument of `fmap', namely
	`(\ arcsHidden -> (\ graph node -> modd arcsHidden graph node))'
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module Folders(
   ) where
data Folder = Folder 

newtype SB x = SB x
newtype SS x = SS x 

instance Functor SS

data NodeArcsHidden = NodeArcsHidden

class HasSS hasS x | hasS -> x where
   toSS :: hasS -> SS x

instance HasSS (SB x) x where
   toSS (SB x) = (SS x)

class HMV option graph node where
   modd :: option -> graph -> node value -> IO ()

instance HMV NodeArcsHidden graph node => HMV (Maybe NodeArcsHidden) graph node


gn :: 
   HMV NodeArcsHidden graph node 
   => graph 
   -> SS (graph -> node Int -> IO ())
gn graph =
   fmap
         (\ arcsHidden -> 
            (\ graph node -> modd arcsHidden graph node))
           (toSS (error "C" :: SB (Maybe NodeArcsHidden)))