Definition of hidden instance members (bug in GHC or Hugs+Yhc)

Hi,

This is either a GHC bug, or a Yhc+Hugs bug - I'm not sure which, but
the compilers disagree:

import Prelude hiding ((==))
data Foo = Foo
instance Eq Foo where
    (==) a b = True

GHC says:
Temp.hs:14:4: `==' is not a (visible) method of class `Eq'

Yhc and Hugs both successfully compile the module.

Does anyone know which compiler(s) are in the wrong, and need bugs filing?

Thanks

Neil