Redundant import warning is sometimes not correct

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.

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture