Bug in context reduction with overlapping instances

[John Hughes]

Here's a small program defining a collection class:
------------------
class Sat a where=20
  dict :: a

data EqD a =3D EqD {eq :: a->a->Bool}

instance Sat (EqD a) =3D> Eq a where
  (=3D=3D) =3D eq dict

class Collection c cxt | c -> cxt where
  empty :: Sat (cxt a) =3D> c a
  single :: Sat (cxt a) =3D> a -> c a
  union :: Sat (cxt a) =3D> c a -> c a -> c a
  member :: Sat (cxt a) =3D> a -> c a -> Bool

instance Collection [] EqD where
  empty =3D []
  single x =3D [x]
  union =3D (++)
  member =3D elem
------------------
Loading this program with overlapping instances turned on produces the =
error message:

Type checking
ERROR "C:\windows\Skrivbord\HugsBug.hs":24 - Cannot justify constraints =
in instance member binding
*** Expression    : member
*** Type          : (Collection [] EqD, Sat (EqD a)) =3D> a -> [a] -> =
Bool
*** Given context : (Collection [] EqD, Sat (EqD a))
*** Constraints   : Eq a

Very strange, given the instance declaration

    instance Sat (EqD a) =3D> Eq a

The behaviour is the same in the February and December 2001 versions.

John Hughes