#1954: Incorrect "Defined but not used" warning when using
GeneralizedNewtypeDeriving
-------------------------+--------------------------------------------------
Reporter: magnus | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.1
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1954>
GHC <http://www.haskell.org/ghc/>
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