GADTs and functional dependencies in ghc 6.10.1

[Reid Barton]

Hello all,

I think (hope) this question is different from the ones about GADTs
recently discussed on this list.  The following program compiles under
ghc 6.8.2 but not under ghc 6.10.1:

> {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, KindSignatures, ScopedTypeVariables #-}
> 
> class Foo a fa | a -> fa where
>   n :: a -> Int
>   
> data Bar :: * -> * -> * where
>   Id :: Bar a a
> 
> baz :: forall a fa b fb. (Foo a fa, Foo b fb) => Bar a b -> Int
> baz Id = n (undefined :: a)

ghc 6.10.1's error message:

/tmp/fundep.hs:10:0:
    Couldn't match expected type `fb' against inferred type `fa'
      `fb' is a rigid type variable bound by
           the type signature for `baz' at /tmp/fundep.hs:9:21
      `fa' is a rigid type variable bound by
           the type signature for `baz' at /tmp/fundep.hs:9:16
    When using functional dependencies to combine
      Foo a fa, arising from a use of `n' at /tmp/fundep.hs:10:9-26
      Foo a fb,
        arising from the type signature for `baz' at /tmp/fundep.hs:10:0-26
    When generalising the type(s) for `baz'

I find the message about the `Foo a fb' constraint quite confusing.
Can anyone explain this error message to me?

If I change the type of Id to Id :: Bar a b, then the program compiles.

Regards,
Reid