warn-unused-imports does not detect coercions
coerce, GHC can erroneously warn that imports are unnecessary. For example:
module Foo (MT(..), M) where import Control.Monad.Identity newtype MT m b = MT (m b) type M b = MT Identity b
import Control.Monad.Identity import Data.Coerce import Foo foo :: M [a] -> MT  a foo = coerce
Main.hs:1:1: Warning: The import of ‘Control.Monad.Identity’ is redundant except perhaps to import instances from ‘Control.Monad.Identity’ To import instances alone, use: import Control.Monad.Identity()
But this is incorrect: we need the constructor
Identity to be in scope in order for the coercion to succeed. Thus the import is not redundant, nor is it needed to (only) import instances. In particular, the two implied suggestions
- remove the import in Main.hs altogether
- change the import in Main.hs to
import Control.Monad.Identity ()
both (correctly) result in compile errors.
At a minimum, the warning message should be updated to include this possibility, but it would be even better if GHC could detect this and not throw a warning at all.