Regression: Detection of unused imports is imprecise (since GHC 8.0.1)

GHCs detection of unused imports used to be very precise. But apparently this changed with GHC 8.0.1. I already noticed for a while that there is something fishy going on and finally came up with a minimal example.

Steps to reproduce:

-- Bar.hs
module Bar (bar, module Control.Monad) where

import Control.Monad

bar :: Integer
bar = 23

-- Foo.hs
module Foo where

import Bar
import Control.Monad (forM_)

foo :: Monad m => [a] -> (a -> m ()) -> m ()
foo = forM_

baz :: Integer
baz = bar

$ ghci -Wall -Werror Foo.hs

• *Expected result:** The program is reject.
• *Actual result:** The program was reject by GHC < 8.0.1, however later versions of GHC accept the program.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture