[Haskell-cafe] problem with type equality constraints

[Ganesh Sittampalam]

Hi,

When I try to compile this code with ghc-6.9.20080310:

module Test2 where

type family Id a
type instance Id Int = Int
type instance Id (a, b) = (Id a, Id b)

class Id a ~ ida => Foo a ida

instance Foo Int Int
instance (Foo a ida, Foo b idb) => Foo (a, b) (ida, idb)

I get these errors:

Test2.hs:12:0:
     Couldn't match expected type `ida' against inferred type `Id a'
       `ida' is a rigid type variable bound by
             the instance declaration at Test2.hs:12:16
     When checking the super-classes of an instance declaration
     In the instance declaration for `Foo (a, b) (ida, idb)'

Test2.hs:12:0:
     Couldn't match expected type `idb' against inferred type `Id b'
       `idb' is a rigid type variable bound by
             the instance declaration at Test2.hs:12:27
     When checking the super-classes of an instance declaration
     In the instance declaration for `Foo (a, b) (ida, idb)'

It seems to me that since Foo a ida and Foo b idb are 
superclassess, Id a ~ ida and Id b ~ idb should be known and so this 
should have worked - am I missing something?

Cheers,

Ganesh