warning about `import Control.Monad.Fix` being unused when mdo is used

and the use is exported. This should not be warned about since Control.Monad.Fix is supposed to be imported in modules that use mdo, to be portable or something. "You should import Control.Monad.Fix. (Note: Strictly speaking, this import is required only when you need to refer to the name MonadFix in your program, but the import is always safe, and the programmers are encouraged to always import this module when using the mdo-notation.)"

{-# OPTIONS_GHC -fglasgow-exts -fwarn-unused-imports #-}

module Test where

import Control.Monad.Fix

isEven :: Int -> Maybe Int
isEven n = if even n then Just n else Nothing

puzzle :: [Int]
puzzle = mdo (x, z) <- [(y, 1), (y^2, 2), (y^3, 3)]
             Just y <- map isEven [z+1 .. 2*z]
             return (x + y)

produces

Test.hs:5:0:
    Warning: Module `Control.Monad.Fix' is imported, but nothing from it is used,
               except perhaps instances visible in `Control.Monad.Fix'
             To suppress this warning, use: import Control.Monad.Fix()

Just doing import Control.Monad.Fix() wouldn't fulfill the purpose of importing that module!

I wonder if there are any other special syntaxes (arrows maybe?) with this effect.

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown