Overlapping instances in Haskell

Question

Reading the book Real world Haskell geting below example of Overlapping instances

instance (JSON a) => JSON [a] where
    toJValue = undefi         


        

Answer 1

These exist

ghci> :i ToJSON
...
instance ToJSON [Char]
...
instance (ToJSON a, ToJSON b) => ToJSON (a, b)

So there'd be an overlap even if GHC took context into account (see Daniel Fischer's answer).