Incorrect "Defined but not used" warning when using GeneralizedNewtypeDeriving

[magnus]

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.

[igloo]

Thanks for the report

[simonpj]

See also #1982

[NeilMitchell]

I have encountered this bug a few times - there is no simple workaround.

[NeilMitchell]

For the person who reaches this bug next, -fno-warn-unused-binds disables this warning, without having to go for -w.

[simonpj]

Thanks for the reminder. It was pretty easy to fix

Tue Mar  9 09:35:55 PST 2010  simonpj@microsoft.com
  * Fix Trac #1954: newtype deriving caused 'defined but not used' error
  
  We were getting a bogus claim that a newtype "data constructor" was
  unused.  The fix is easy, although I had to add a field to the constructor
  TcEnv.NewTypeDerived
  
  See Note [Newtype deriving and unused constructors] in TcDeriv

    M ./compiler/typecheck/TcDeriv.lhs -3 +25
    M ./compiler/typecheck/TcEnv.lhs -8 +15
    M ./compiler/typecheck/TcInstDcls.lhs -1 +1

Could be merged to 6.12, but not if it causes any trouble.

Simon

[igloo]

Merged