Hello,

I tried to compile my project with GHC 8 (from HEAD).

And I got

    • Overlapping instances for aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON
...
        arising from a use of ‘serve’
      Matching instances:
        two instances involving out-of-scope types
          instance aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON b =>
                   aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON
                     (Data.Tagged.Tagged a b)
            -- Defined in ‘aeson-0.11.1.0:Data.Aeson.Types.Instances’
          instance forall k x (r :: k).
                   aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON x =>
                   aeson-0.11.1.0:Data.Aeson.Types.Class.ToJSON
                     (Data.Tagged.Tagged r x)

One of instances should be removed probably. 

I suppose that instance for removing should be instance in Data.Tagged. 

But version in Aeson should be PolyKinded.

What do you think?

Best regards,

Dmitry 

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