Class deriving in GHC 6.8

Hello all!

How come in GHC 6.6 I could to write

> {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}
> data Foo   = Foo         deriving Show
> data Bar c = Bar (c Foo) deriving Show

but in GHC 6.8.2 I get the error

>     No instance for (Show (c Foo))
>       arising from the 'deriving' clause of a data type declaration
>                    at Ert.hs:3:0-37
>     Possible fix: add an instance declaration for (Show (c Foo))
>     When deriving the instance for (Show (Bar c))

Thanks,

/ Emil