[GHC] #4103: report location of global package db in ghc --info

[GHC]

#4103: report location of global package db in ghc --info
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    Reporter:  duncan            |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.2            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
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 Cabal does a sanity check to make sure it has a ghc and ghc-pkg program
 that are the same version. This is sometimes not enough.

 In #haskell today we had a case where there was `/usr/bin/ghc` and
 `/usr/local/bin/ghc-pkg`. This gave rise to pretty weird behaviour that
 was hard to track down. Packages could be built, but when they were
 installed they always appeared to be broken, missing a dependency on base.

 We could improve Cabal's sanity check if it checked that `ghc` and `ghc-
 pkg` agreed on the location of the global package database. That would
 seem to be the canonical check that the two belong to a single instance of
 a ghc installation.

 To be able to perform this check, it would be useful to extend ghc's
 `--info` flag to include the location of the global package db.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4103>
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