Hi,
The following program incorrectly produces a warning about an unused import :

module Bug ( Structure (..) ) where

import Data.Ratio ( Rational )

data Structure a where
  StructCons  :: Int   -> Structure Int
  StructRatio ::          Structure Rational


produces:


Bug.hs:3:0:
   Warning: Module `Data.Ratio' is imported, but nothing from it is used,
              except perhaps instances visible in `Data.Ratio'
            To suppress this warning, use: import Data.Ratio()

Bug.hs:3:20:
   Warning: Imported from `Data.Ratio' but not used:
              type constructor or class `Rational'


with ghc checked-out from the HEAD 4 days ago (but the problem has been present for quite some time).

ghc-6.4.1, rightly, does not produce any message.

Of course the problem exists with any import, not just Rational.

Alain

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