MINIMAL pragma should supprt negation

Consider this class declaration

class Conv a where
    to :: Integer -> a
    from :: a -> Integer
    default to :: (Generic a) => Integer -> a
    to i = ...
    default from :: (Generic a) => a -> Integer
    from a = ...

The class provides default methods for the two methods using generics. An instance declaration for this type is likely to want to use the default for both methods or for none. So I'd like a MINIMAL pragma that can express this. E.g.

    {-# MINIMAL (to, from) | (!to, !from) #-}

I've used ! for negation (following the lead set by the MINIMAL pragma using a non-Haskell OR operator), so this says: either implement 'to' and 'from' or don't implement neither 'to' nor 'from'.

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Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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That’s no longer a statement of what instances are MINIMAL, but rather a complete description of sensible instantiations, isn’t it?

Yes, you could say that. Implementing nothing is MINIMAL, but if you implement to you must also implement from (and v.v.). But I think it's a natural extension of the MINIMAL pragma.

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