Overzealous unused-top-binds

The following example produces three warnings:

module Foo () where

type A = Int
type B = A

foo :: IO ()
foo = print (bar 3 :: B)

bar :: a -> a
bar = id

ghci Foo -Wall produces

GHCi, version 8.0.1.20161117: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Foo              ( Foo.hs, interpreted )

Foo.hs:5:1: warning: [-Wunused-top-binds]
    Defined but not used: type constructor or class ‘B’

Foo.hs:8:1: warning: [-Wunused-top-binds]
    Defined but not used: ‘foo’

Foo.hs:11:1: warning: [-Wunused-top-binds]
    Defined but not used: ‘bar’
Ok, modules loaded: Foo.

The three errors reported are inconsistent because GHC reports that B is unused, but somehow feels that A *is* used.

It seems to me that two possibilities could work:

1. GHC reports 1 error:

GHC should only report that foo is unused, since A is used in B, B is used in foo, and bar is used in foo. This seems like the best approach to me.

1. GHC reports 4 errors:

One might choose to apply the definition of "unused" transitively so that, since foo is unused, everything used in foo is considered unused (unless "used" elsewhere). In this case, A should also be reported as unused, but it currently isn't. This definition would be exceedingly useful for removing dead code, but it also produces very noisy output when there is a lot of dead code.

Trac metadata

Trac field	Value
Version	8.0.2-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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