Re: [GHC] #2800: deprecated OPTIONS flag warnings generated during dep chasing?

[GHC]

#2800: deprecated OPTIONS flag warnings generated during dep chasing?
-+--
Reporter:  duncan|Owner:  
Type:  bug   |   Status:  closed  
Priority:  normal|Milestone:  6.12 branch 
   Component:  Compiler  |  Version:  6.8.3   
Severity:  normal|   Resolution:  wontfix 
Keywords:|   Difficulty:  Unknown 
Testcase:|   Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
-+--
Changes (by igloo):

  * status:  new => closed
  * resolution:  => wontfix

Comment:

 The only reason putting `-fno-warn-deprecated-flags` in the Cabal file
 doesn't work is because Cabal is a special case, and wants to be
 bootstrappable with just `ghc --make`. I think we should just live with a
 couple of warnings in this case, rather than going to lots of effort to
 work around the problem.

 Also, the current warnings at least will stop being a problem as support
 for building with older GHC versions is dropped.

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Re: [GHC] #2800: deprecated OPTIONS flag warnings generated during dep chasing?

[GHC]

#2800: deprecated OPTIONS flag warnings generated during dep chasing?
-+--
Reporter:  duncan|Owner:  
Type:  bug   |   Status:  new 
Priority:  normal|Milestone:  6.12 branch 
   Component:  Compiler  |  Version:  6.8.3   
Severity:  normal|   Resolution:  
Keywords:|   Difficulty:  Unknown 
Testcase:|   Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
-+--
Changes (by igloo):

  * difficulty:  => Unknown
  * milestone:  => 6.12 branch

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[GHC] #2800: deprecated OPTIONS flag warnings generated during dep chasing?

[GHC]

#2800: deprecated OPTIONS flag warnings generated during dep chasing?
-+--
Reporter:  duncan|   Owner:  
Type:  bug   |  Status:  new 
Priority:  normal|   Component:  Compiler
 Version:  6.8.3 |Severity:  normal  
Keywords:|Testcase:  
Architecture:  Unknown/Multiple  |  Os:  Unknown/Multiple
-+--
 It would appear that warnings about deprecated flags used in `OPTIONS`
 pragmas get spat out during the early pre-processing and module chasing
 phase when they would not be generated after pre-processing.

 `ghc --make` has to scan each module to look for options and language
 extensions to know if it has to run cpp. At this stage it cannot of course
 run cpp so it does not necessarily get the full set of options and
 extensions since some may be conditional. So it should not at this stage
 generate any warnings since they may be suppressed by later cpp-
 conditional flags.

 Another possible theory is that it generates the warnings before having
 read all the `OPTIONS` flags so the later warning suppression is
 ineffective. Unfortunately we have to put them in the order we do for
 compatibility with older versions of ghc.

 Here's an example out of Cabal:

 {{{
 {-# OPTIONS -cpp -fffi #-}
 -- OPTIONS required for ghc-6.4.x, and must appear first
 {-# LANGUAGE CPP, ForeignFunctionInterface #-}
 {-# OPTIONS_GHC -cpp -fffi #-}
 {-# OPTIONS_NHC98 -cpp #-}
 {-# OPTIONS_JHC -fcpp -fffi #-}
 #if __GLASGOW_HASKELL__ >= 610
 {-# OPTIONS_GHC -fno-warn-deprecated-flags #-}
 -- the ghc flag -fffi is deprecated in ghc-6.10. We're
 -- supposed to use the LANGUAGE pragmas instead. However
 -- we have to maintain compatibility with older ghc versions
 -- so we (try to) suppress this warning.
 #endif
 }}}

 What we're trying to do here is make it work with ghc-6.4 + and not
 generate any warnings with the latest ghc. This seems quite tricky to do.

 To support ghc-6.4 we have to put the `OPTIONS` pragma first, since it
 apparently only looks at the first couple lines. We then use the compiler-
 specific `OPTIONS` pragmas. In particular these are needed for ghc-6.6.
 For ghc-6.8 and later we can use the `LANGUAGE` pragmas.

 In ghc-6.10 the `-fffi` option is deprecated. However we cannot
 conditionally compile the `OPTIONS -cpp -fffi` since ghc needs to see the
 `-cpp` to know that it has to cpp the module. Making the `OPTIONS -cpp`
 unconditional and the `OPTIONS -cpp -fffi` conditional does not work with
 ghc-6.4.

 So instead we try suppressing the warning. We cannot do that
 unconditionally since the flag is only present in newer ghc versions.
 However if we do suppress conditionally then ghc still warns anyway,
 presumably because it is warning on the first pass rather than in the pass
 after running cpp.

 All in all it's a bit tricky to do right. Indeed I cannot currently see a
 way to make it work at all without generating warnings. The sledgehammer
 would be to put a flag in the .cabal file and have it apply to all
 modules, however that does not prevent the warning happening during Cabal
 bootstrapping which uses plain `ghc --make`.

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