Summary

When reexporting symbols from Prelude from some module A, then further reexporting A from another module which also imports the Prelude qualified, -Wdodgy-exports can be triggered. This pattern of reexporting shows up often in "alternate Preludes".

Steps to reproduce

Produce a module Fun:

module Fun
  ( Data.Bool.Bool(..)  -- A symbol also present in Prelude
  ) where

import qualified Data.Bool

now produce a module Lib:

module Lib
  ( module Fun  -- reexports Bool
  ) where

import qualified Fun
-- import qualified Prelude

Compile with -Wall, and see no warnings. Now uncomment the qualified import of Prelude and see the following:

/home/colin/code/haskell/dodgy-bug/src/Lib.hs:2:5-14: warning: [-Wdodgy-exports]
    The export item ‘module Fun’ exports nothing
  |
2 |   ( module Fun
  |     ^^^^^^^^^^
/home/colin/code/haskell/dodgy-bug/src/Lib.hs:5:1-20: warning: [-Wunused-imports]
    The qualified import of ‘Fun’ is redundant
  |
5 | import qualified Fun
  | ^^^^^^^^^^^^^^^^^^^^
/home/colin/code/haskell/dodgy-bug/src/Lib.hs:6:1-24: warning: [-Wunused-imports]
    The qualified import of ‘Prelude’ is redundant
  |
6 | import qualified Prelude
  | ^^^^^^^^^^^^^^^^^^^^^^^^

Now remove the qualified section of the Prelude import, and see no warnings. The presence/absence of NoImplicitPrelude seems to have no effect here.

Expected behavior

I'd expect no warnings in any of the three cases.

Environment

• GHC version used: 8.6.5

Optional:

• Operating System: Arch Linux
• System Architecture: x86_64