Problem with functional dependencies

[Daniel Gorín]

Hi

I have some code that uses MPTC + FDs + flexible and undecidable  
instances that was working fine until I did a trivial modification on  
another part of the project. Now, GHC is complaining with a very  
confusing (for me, at least) error message. I've been finally able to  
reproduce the problem using these three small modules:

 > {-# LANGUAGE MultiParamTypeClasses #-}
 > {-# LANGUAGE FunctionalDependencies #-}
 > {-# LANGUAGE FlexibleInstances #-}
 > module M1
 >
 > where
 >
 > data M n = M
 > data F n = F
 >
 > class C m f n | m -> n, f -> n where
 >     c :: m -> f -> Bool
 >
 > instance C (M n) (F n) n where
 >     c _ _ = True

 > {-# LANGUAGE MultiParamTypeClasses #-}
 > {-# LANGUAGE UndecidableInstances #-}
 > {-# LANGUAGE FlexibleInstances #-}
 > module M2
 >
 > where
 >
 > import M1
 >
 > newtype F'= F' (F N)
 >
 > data N = N
 >
 > instance C m (F N) N => C m F' N where
 >      c m (F' f) = c m f

 > module M3
 >
 > where
 >
 > import M1
 > import M2()
 >
 > data N' = N'
 >
 > go :: M N' -> F N' -> Bool
 > go m f = c m f

Now, when trying to compile M3 (both in 6.6.1 and 6.8.1) I get:

M3.hs:11:0:
     Couldn't match expected type `N'' against inferred type `M2.N'
     When using functional dependencies to combine
       C m M2.F' M2.N, arising from the instance declaration at M2.hs: 
13:0
       C (M N') (F N') N', arising from use of `c' at M3.hs:11:9-13
     When generalising the type(s) for `go'

It is worth observing that:

- M2 compiles fine
- No type defined in M2 is visible in M3
- if the "import M2()" is commented out from M3, it compiles fine
- if, in M3, N' is placed by N (needs to be imported), everything  
compiles again

Normally, it takes me some time to digest GHC's type-classes-related  
error messages, but after some reflection, I  finally agree with  
them. This time, however, I'm totally lost. I can't see any reason  
why N' and M2.N would have to be unified, nor why this code should be  
rejected.

Any help would be much appreciated!

Daniel