Clarify documentation of -Wdeprecations vs -Wwarnings-deprecations

The documented and official flag for the warning when a module, function or type with a WARNING or DEPRECATED pragma is used is -fwarn-warnings-deprecations. (See for instance https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/options-sanity.html). If the goal is turn off the warning I use -fno-warn-warnings-deprecations flag. By default -fwarn-warnings-deprecations on.

In the next example shows a warning by using the flag mention above.

$ cat Foo.hs
{-# OPTIONS_GHC -fwarn-warnings-deprecations #-}
import Data.Version (Version(Version, versionBranch, versionTags))
main :: IO ()
main = do
  let v = Version {
          versionBranch = [1,0,0]
        , versionTags = ["beta"]
        }
  return ()

and GHC outputs:

$ ghc Foo.hs
[1 of 1] Compiling Main             ( Foo.hs, Foo.o )

Foo.hs:9:11: warning: [-Wdeprecations]
    In the use of ‘versionTags’ (imported from Data.Version):
    Deprecated: "See GHC ticket #2496"
Linking Foo ...

The first unexcepted behavior is the warning message showed above.

Following the convention the warning message should be:

Foo.hs:9:11: warning: [-Wwarnings-deprecations]

The second unexcepted behavior happened when I used a pragma to silenced the warning warnings-deprecations, I committed the mistake to misspelled it, but surprisingly GHC didn't complaint about the unknown pragma, it seems treated again like a synonymous of -fno-warn-warnings-deprecations in this case.

{-# OPTIONS_GHC -fno-warn-deprecations  #-}
import Data.Version (Version(Version, versionBranch, versionTags))
main :: IO ()
main = do
  let v = Version {
          versionBranch = [1,0,0]
        , versionTags = ["beta"]
        }
  return ()

Searching a little, I found it is not documented the flag -Wdeprecations in recent versions of GHC ( I tested with 7.8.2 too) but it does for instance available in GHC 6.3.0 (See https://manned.org/ghc-cvs/42a4c961) but it actually continues appearing in the source code of the recent GHC. See https://github.com/mlen/ghc/search?utf8=%E2%9C%93&q=deprecations.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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CC	asr
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