MINIMAL pragma and default implementations

The ToJSON class from the aeson library recently gained the new method toEncoding which has a default implementation based on the existing toJSON method.

A MINIMAL pragma was added to toJSON to make sure instances will define it.

However, toJSON also has a default implementation but only if the type has an instance for Generic:

class ToJSON a where
    toJSON     :: a -> Value
    {-# MINIMAL toJSON #-}

    default toJSON :: (Generic a, GToJSON (Rep a)) => a -> Value
    toJSON = genericToJSON defaultOptions

    toEncoding :: a -> Encoding
    toEncoding = Encoding . E.encodeToBuilder . toJSON

The problem is that users of aeson who are using the generic implementation get warnings that their toJSON method is not defined:

No explicit implementation for
  ‘toJSON’
In the instance declaration for ‘ToJSON MyType’

It would be nice if GHC is a bit smarter in this case so that when a a default implementation is used it won't give the warning.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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