[GHC] #13064: Incorrect redudant imports warning

[GHC]

#13064: Incorrect redudant imports warning
-------------------------------------+-------------------------------------
        Reporter:  phadej            |                Owner:  davide
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15393            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by davide:

Old description:

> With a simple package:
>
> issue.cabal:
> {{{
> name: issue
> version: 0
> cabal-version: >= 1.10
> build-type: Simple
>
> library
>   build-depends: base, binary, binary-orphans==0.1.5.2
>   exposed-modules: Issue
> }}}
>
> Issue.hs:
> {{{
> {-# LANGUAGE DeriveGeneric #-}
> {-# OPTIONS_GHC -Wall #-}
> module Issue (T (..)) where
>
> import Prelude ()
> import Data.Binary.Orphans
> import Data.Binary (Binary (..))
> import GHC.Generics (Generic)
> import Data.Aeson (Value)
>
> data T = T Value deriving (Generic)
>
> instance Binary T
> }}}
>
> GHC 7.10.3 correctly reports:
> {{{
> Issue.hs:7:1: Warning:
>     The import of ‘Data.Binary’ is redundant
>       except perhaps to import instances from ‘Data.Binary’
>     To import instances alone, use: import Data.Binary()
> }}}
>
> GHC 8.0.1 incorrectly (!!!) reports:
>
> {{{
> Issue.hs:6:1: warning: [-Wunused-imports]
>     The import of ‘Data.Binary.Orphans’ is redundant
>       except perhaps to import instances from ‘Data.Binary.Orphans’
>     To import instances alone, use: import Data.Binary.Orphans(
> }}}
>
> but `Binary Value` instance is imported from `Data.Binary.Orphans`.
>
> ---
>
> In real life while compiling https://github.com/futurice/haskell-mega-
> repo/blob/93c3f111f39c973769c35725d90c9b8ef9a57de3/futurice-
> github/src/Futurice/GitHub.hs the `Data.Binary` redundant import isn't
> reported, as `Futurice.Prelude` exports other stuff, which is  used. See
> https://gist.github.com/phadej/bb26df19c611260ab8f867729def39b9 for
> minimal imports reported with `-ddump-minimal-imports`.

New description:

 With a simple package:

 issue.cabal:
 {{{
 name: issue
 version: 0
 cabal-version: >= 1.10
 build-type: Simple

 library
   build-depends: base, binary, binary-orphans==0.1.5.2
   exposed-modules: Issue
 }}}

 Issue.hs:
 {{{
 {-# LANGUAGE DeriveGeneric #-}
 {-# OPTIONS_GHC -Wall #-}
 module Issue (T (..)) where

 import Prelude ()
 import Data.Binary.Orphans
 import Data.Binary (Binary (..))
 import GHC.Generics (Generic)
 import Data.Aeson (Value)

 data T = T Value deriving (Generic)

 instance Binary T
 }}}

 GHC 7.10.3 correctly reports:
 {{{
 Issue.hs:7:1: Warning:
     The import of ‘Data.Binary’ is redundant
       except perhaps to import instances from ‘Data.Binary’
     To import instances alone, use: import Data.Binary()
 }}}

 GHC 8.0.1 incorrectly (!!!) reports:

 {{{
 Issue.hs:6:1: warning: [-Wunused-imports]
     The import of ‘Data.Binary.Orphans’ is redundant
       except perhaps to import instances from ‘Data.Binary.Orphans’
     To import instances alone, use: import Data.Binary.Orphans(
 }}}

 but `Binary Value` instance is imported from `Data.Binary.Orphans`.

 ---

 In real life while compiling https://github.com/futurice/haskell-mega-
 repo/blob/93c3f111f39c973769c35725d90c9b8ef9a57de3/futurice-
 github/src/Futurice/GitHub.hs the `Data.Binary` redundant import isn't
 reported, as `Futurice.Prelude` exports other stuff, which is  used. See
 https://gist.github.com/phadej/bb26df19c611260ab8f867729def39b9 for
 minimal imports reported with `-ddump-minimal-imports`.

 ## Status

 I've submitted some PRs to fix the impending unused import warnings:

 * https://github.com/haskell/cabal/pull/5673
 * https://github.com/kolmodin/binary/pull/159
 * MERGED https://github.com/haskell/bytestring/pull/168
 * https://github.com/haskell/containers/pull/576
 * MERGED https://github.com/haskell/text/pull/240

 I've also added a new ghc patch here:
 * https://phabricator.haskell.org/D5312

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13064#comment:44>
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