How about a "deriving defaulted" deriving strategy

[Andrew Martin]

I was just doing the usual incantation for an exception type:

    data DatagramException = ...
      deriving stock (Eq,Show)
    instance Exception Datagram

And it suddenly hit me what a shame it was that I cannot put the Exception
instance in a deriving clause like the others. It would be more visually
pleasing to write:

    data DatagramException = ...
      deriving stock (Eq,Show)
      deriving defaulted (Exception)

-- 
-Andrew Thaddeus Martin
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