Generics deriving is quadratic

I'm compiling some code with many sum type alternatives and it's absurdly slow.

{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics (Generic)

data D = D1 | D2 | ... | D400 deriving (Generic)

main = return ()

I did a little benchmark and it's actually precisely O(n²) in the number of alternatives.

Easy repro: https://github.com/nh2/ghc-generics-deriving-is-slow/

I assume that this is a bug and it should be linear.

Note that it is also quadratic in memory usage.