Incorrect redudant imports warning

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:

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

I've also added a new ghc patch here:

• MERGED https://phabricator.haskell.org/D5312