[GHC] #13919: Incorrect unused top binding warning when interacting with GHC.Generics

[GHC]

#13919: Incorrect unused top binding warning when interacting with GHC.Generics
-------------------------------------+-------------------------------------
           Reporter:  Taneb          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The program
 {{{#!hs
 {-# 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

 {{{#!hs
 {-# 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"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13919>
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