module containing GADTs no longer compiles in ghc 6.8.0

[Daniel Gorín]

Hi

I just tried to compile a project of mine that builds fine using ghc 6.6.1 and 
got many errors like this:

src/HyLo/Formula/NNF.hs:247:48:
    GADT pattern match in non-rigid context for `Opaque'
      Tell GHC HQ if you'd like this to unify the context
    In the pattern: Opaque f'
    In the expression: \ (Opaque f') -> Opaque (Box r f')
    In the definition of `box':
        box = \ (Opaque f') -> Opaque (Box r f')

I don't know what a "non-rigid context" is, nor if "I like this to unify the 
context" or not, but I would certainly be happy if I could get this module to 
compile again! :)

For the record, I was using ghc-6.8.0.20070917. Please let me know if you need 
further information....

Thanks
Daniel

PS: On a side note, I found this error message to be kind of funny. It seems 
to indicate no real error but some sort of error-message-driven poll!