[GHC] #7779: building GHC overwrites the installed package database if GHC_PACKAGE_PATH is set

[GHC]

#7779: building GHC overwrites the installed package database if GHC_PACKAGE_PATH
is set
---------------------------------+------------------------------------------
    Reporter:  heatsink          |       Owner:  igloo           
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:  7.8.1           
   Component:  Build System      |     Version:  7.4.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
---------------------------------+------------------------------------------

Comment(by heatsink):

 I expect building software to pose no risk of breaking any installed
 software.  It is understandable that installing software may break other
 software, but building should not.

 I've been using `GHC_PACKAGE_PATH` to install two versions of GHC that
 both call themselves `ghc-7.4.2`.  By default, they both look in the same
 user package database, which breaks at least one of the two versions
 because packages are not compatible with both GHCs.  Setting
 `GHC_PACKAGE_PATH` fixes that problem.  Cabal works if given options
 `--global --package-db=$FOO`.  If this way of managing paths is
 deprecated, then I'd like to have an alternative.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7779#comment:3>
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