[Haskell-cafe] Reverse Show instance

[Andrew Coppin]

   Cannot deduce (Show x) from context (Show (x, y)).
   Cannot deduce (Show y) from context (Show (x, y)).

Um... seriously?

 From Prelude, we have

   Show x, Show y => Show (x, y)

So clearly it works in the forward direction. But apparently not in the 
reverse direction.

Is this a bug or a feature? (I.e., is there some obscure possibility I 
haven't thought of which means that doing the reverse inference would be 
incorrect?)