[GHC] #12056: Too aggressive `-w` option

[GHC]

#12056: Too aggressive `-w` option
-------------------------------------+-------------------------------------
        Reporter:  asr               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11429, #11789    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by thomie):

 > It looks like this only happens when -w is combined with
 `-Wunrecognised-warning-flags`.

 To be clear, what I meant by that is that `-w` is too aggressive only when
 placed before `-Wunrecognised-warning-flags`. When `-w` is combined with
 other (existing) `-W` flags, GHC does the right thing (take the order in
 which the flags are passed into account).

 Here is an example, with `Test.hs` containing:
 {{{
 module Test where
 import System.FilePath
 }}}

 * TEST `-w` BEFORE.

 `-w` disables all warning flags, but since we supply `-Wunused-imports`
 afterwards, we *do* get a warning:
 {{{
 $ ghc -w -Wunused-imports -fforce-recomp Test.hs
 [1 of 1] Compiling Test             ( Test.hs, Test.o )

 Test.hs:2:1: warning: [-Wunused-imports]
     The import of ‘System.FilePath’ is redundant
       except perhaps to import instances from ‘System.FilePath’
     To import instances alone, use: import System.FilePath()
 }}}


 * TEST `-w` AFTER
 We give `-Wunused-imports` first. But since we give `-w` afterwards, which
 reverts the previous flag, we do *not* get a warning:
 {{{
 $ ghc -Wunused-imports -w -fforce-recomp Test.hs
 }}}

 Both are expected behavior.

 == Question
 What should happen in the following example:
 {{{
 ghc -Wfoo1 -w -Wfoo2 -Wunrecognised-warning-flags -Wfoo3 Main.hs
 }}}

 == Option 1
 * warn about `-Wfoo` (since `-Wunrecognised-warning-flags` is on by
 default)
 * not warn about `-Wfoo2` (since it comes after `-w`)
 * warn about `-Wfoo3` (since it comes after `-Wunrecognised-warning-
 flags`)

 == Option 2
 * warn about all `-Wfoo*`, since `-Wunrecognised-warning-flags` came after
 `-w`, so `-Wunrecognised-warning-flags` is enabled

 wozgonon: thanks for working on this! I think all these flags are Dynamic
 though, so I'm not sure that `Note` is relevant. The text "on the
 commandline" is coming from `ghc/Main.hs` most likely.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12056#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler