[GHC] #12067: warn-unused-imports does not detect coersions

[GHC]

#12067: warn-unused-imports does not detect coersions
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10347            |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by crockeea:

@@ -34,2 +34,2 @@
- nor is it needed to import instances. In particular, the two implied
- suggestions
+ nor is it needed to (only) import instances. In particular, the two
+ implied suggestions
@@ -37,2 +37,2 @@
-  1. remove the import altogether
-  2. change the import to `import Control.Monad.Identity`
+  1. remove the import in Main.hs altogether
+  2. change the import in Main.hs to `import Control.Monad.Identity ()`

New description:

 When using `coerce`, GHC can erroneously warn that imports are
 unnecessary. For example:

 Foo.hs
 {{{
 module Foo (MT(..), M) where

 import Control.Monad.Identity

 newtype MT m b = MT (m b)
 type M b = MT Identity b
 }}}

 Main.hs
 {{{
 import Control.Monad.Identity
 import Data.Coerce
 import Foo

 foo :: M [a] -> MT [] a
 foo = coerce
 }}}

 GHC (with `-fwarn-unused-imports`) warns
 {{{
 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

  1. remove the import in Main.hs altogether
  2. 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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12067#comment:2>
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