TF (juick : http://juick.com/segfault/2863032#5)

lol.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}

import Data.Time

type family TimeDiff a where
  TimeDiff UTCTime = NominalDiffTime
  TimeDiff a = a

class Diffable a {-| a -> Diff a-} where
  diff :: a -> a -> TimeDiff a
  addDiff :: TimeDiff a -> a -> a

instance {-# OVERLAPPABLE #-} (Num a, TimeDiff a ~ a) => Diffable a where
  diff = (-)
  addDiff = (+)

instance {-# OVERLAPPING #-} Diffable UTCTime where
  diff = diffUTCTime
  addDiff = addUTCTime


-- Compiles.

{-
import Data.Time

class Diffable a where
  type Diff a :: *
  diff :: a -> a -> Diff a
  addDiff :: Diff a -> a -> a

instance {-# OVERLAPPABLE #-} (Num a) => Diffable a where
  type Diff a = a
  diff = (-)
  addDiff = (+)

instance Diffable UTCTime where
  type Diff UTCTime = NominalDiffTime
  diff = diffUTCTime
  addDiff = addUTCTime

--    Conflicting family instance declarations:
--      Diff a -- Defined at src/Peridot/Diffable.hs:11:8
--      Diff UTCTime -- Defined at src/Peridot/Diffable.hs:16:8
-}