[GHC] #12920: Overzealous unused-top-binds

[GHC]

#12920: Overzealous unused-top-binds
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 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.

 2. 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.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12920>
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