Incorrect "Defined but not used" warning when using GeneralizedNewtypeDeriving

When compiling

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# OPTIONS_GHC -Wall -Werror #-}
module Bug(P, runP) where

import Control.Monad.Identity(Identity, runIdentity)

newtype P a = P { unP :: Identity a } deriving Monad

runP :: P a -> a
runP = runIdentity . unP

I get

Bug.hs:7:14: Warning: Defined but not used: data constructor `P'

although the constructor is used in the derived Monad instance. This is obviously not a show stopper, but it forces me to rewrite what to me looks like an OK program if I want to stick to the given OPTIONS_GHC.

Trac metadata

Trac field	Value
Version	6.8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown