[Hackage] #490: cabal unpack gives bad error message for unsatisfiable constraints

[Hackage]

#490: cabal unpack gives bad error message for unsatisfiable constraints
----------------------------------+-----------------------------------------
  Reporter:  duncan               |        Owner:                   
      Type:  defect               |       Status:  new              
  Priority:  normal               |    Milestone:  cabal-install-0.6
 Component:  cabal-install tool   |      Version:  1.6.0.1          
  Severity:  normal               |     Keywords:                   
Difficulty:  very easy (<1 hour)  |   Ghcversion:                   
  Platform:                       |  
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 The simple use case works fine:

 {{{
 $ cabal unpack HaXml
 Downloading HaXml-1.13.3...
 Unpacking HaXml-1.13.3...
 }}}

 But we also allow constraints and that does not necessarily work:

 {{{
 $ cabal unpack HaXml-1.19.6
 cabal: List.maximumBy: empty list
 }}}

 The problem is that the index does not contain `HaXml-1.19.6`.  After a
 `cabal update` it is fine. In general this would happen for any user-
 supplied version constraint that turns out to be unsatisfiable.

 The function `resolvePackages` in `Distribution.Client.Unpack` does not
 consider the possibility that a dependency may be unsatisfiable.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/490>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects