internal error caused by adding an instance to a type class with a functional dependency and a default method

I've got trouble with the following program:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
class Foo a b | a -> b where
    foo :: a -> b
    foo = undefined

instance Foo a (Maybe a)

This results in a type checking error, and an internal error:

xx.hs:2:0:
    Couldn't match expected type `b' (a rigid variable)
           against inferred type `Maybe a'
      `b' is bound by the class declaration for `Foo'
    When using functional dependencies to combine
      Foo a (Maybe a), arising from the instance declaration at x.hs:8:0
      Foo a b,
        arising from is bound by the class declaration for `Foo'
        at x.hs:(2,0)-(4,18)
[snip]
x.hs:6:0:
    GHC internal error: `Main.$dmfoo' is not in scope
    In the expression: Main.$dmfoo
    In the definition of `foo': foo = Main.$dmfoo
    In the definition for method `foo'

Slightly less convincing variations on the scheme are

class Foo a b | a -> b
instance Foo a (Maybe a)

foo :: Foo a b => a -> b
foo = undefined

class Bar a b c

instance (Foo a b, Foo b c) => Bar a b c

Both the definition of foo and the instance declaration of Bar give a type checking error "when combining functional dependencies".

Apparently ghc doesn't use functional dependencies to simplify the contexts of user supplied type signatures; I think the above examples indicate that it should.

Trac metadata

Trac field	Value
Version	6.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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