Incorrect unused top binding warning when interacting with GHC.Generics

The program

{-# LANGUAGE DeriveGeneric #-}
module Main (main) where

import Data.Aeson
import GHC.Generics

data Foo = Foo {bar :: String} deriving Generic

instance ToJSON Foo where
  toJSON = genericToJSON defaultOptions

main :: IO ()
main = print (toJSON (Foo "hello"))

emits the warning

UTB.hs:7:17: warning: [-Wunused-top-binds]
    Defined but not used: ‘bar’

but removing that binding changes the behaviour of the program, as the above prints

Object (fromList [("bar",String "hello")])

but

{-# LANGUAGE DeriveGeneric #-}
module Main (main) where

import Data.Aeson
import GHC.Generics

data Foo = Foo String deriving Generic

instance ToJSON Foo where
  toJSON = genericToJSON defaultOptions

main :: IO ()
main = print (toJSON (Foo "hello"))

prints

String "hello"

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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