[GHC] #3992: Redundant import warning is sometimes not correct

[GHC]

#3992: Redundant import warning is sometimes not correct
---------------------------------+------------------------------------------
    Reporter:  basvandijk        |       Owner:                                   
        Type:  bug               |      Status:  new                              
    Priority:  normal            |   Component:  Compiler                         
     Version:  6.12.1            |    Keywords:                                   
          Os:  Unknown/Multiple  |    Testcase:                                   
Architecture:  Unknown/Multiple  |     Failure:  Incorrect warning at compile-time
---------------------------------+------------------------------------------
 When working on my usb-safe library I just discovered a warning message
 that doesn't seem right.

 Suppose you have the following three modules:

 (I tried to trim down the problem so I simplified and changed most types,
 classes, instances and functions. They don't matter for this discussion.
 The only things that matter are the names that are in- and exported)

 {{{Resource.hs:}}}
 {{{
 {-# LANGUAGE TypeFamilies #-}

 module Resource ( Resource, Handle, open, close ) where

 class Resource r where
     data Handle r :: *

     open  :: r -> IO (Handle r)
     close :: Handle r -> IO ()
 }}}

 {{{Region.hs:}}}
 {{{
 module Region ( open, run ) where

 open :: Char -> String
 open = (:[])

 run :: Maybe a -> Bool
 run = maybe False (const True)
 }}}

 {{{USB.hs:}}}
 {{{
 {-# LANGUAGE TypeFamilies #-}

 module USB ( Device(..)
            , claim
            , foo
            , module Region
            ) where

 import Resource ( Resource, Handle, open, close )
 import Region
 import qualified Region as QualifiedRegion ( open )

 newtype Device = Device {unDevice :: Int}

 instance Resource Device where
     data Handle Device = DeviceHandle Int

     open  = return . DeviceHandle . unDevice
     close = const $ return ()

 claim :: Char -> String
 claim = QualifiedRegion.open

 foo :: Maybe a -> Bool
 foo = run
 }}}

 When I load USB.hs in ghci with all warnings on I get the following
 warning:

 {{{$ ghci -Wall USB.hs}}}
 {{{
 USB.hs:9:0:
     Warning: The import of `Resource.open'
              from module `Resource' is redundant
 Ok, modules loaded: USB, Region, Resource.
 }}}

 Why is {{{Resource.open}}} redundant while I use it in an instance
 declaration?

 If the warning is correct so {{{Resource.open}}} is redundant we can just
 as well remove the import. However when I remove it I get the following
 error:

 {{{
 USB.hs:18:4: `open' is not a (visible) method of class `Resource'
 Failed, modules loaded: Region, Resource.
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3992>
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