[Hackage] #470: identify and exclude broken or unusable packages when constructing and merging package dbs

[Hackage]

#470: identify and exclude broken or unusable packages when constructing and
merging package dbs
---------------------------------+------------------------------------------
  Reporter:  guest               |        Owner:                
      Type:  defect              |       Status:  new           
  Priority:  normal              |    Milestone:                
 Component:  cabal-install tool  |      Version:  1.6.0.1       
  Severity:  normal              |   Resolution:                
  Keywords:                      |   Difficulty:  hard (< 1 day)
Ghcversion:                      |     Platform:                
---------------------------------+------------------------------------------
Changes (by duncan):

  * summary:  bad error message when a package transtively depends on two
              versions of the same one => identify and
              exclude broken or unusable packages when
              constructing and merging package dbs

Comment:

 We should identify these broken and unusable packages when constructing
 and merging package dbs. Then when solving we should add exclude
 constraints on such packages. If those packages are then needed then we
 should report the reason they were excluded.

 It may also be worth warning whenever we detect this situation at all,
 even if we do not need the packages in question. Something like `ghc-pkg
 check` perhaps to do a global package consistency check.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/470#comment:2>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects