[Hackage] #694: mention --constraint flag in error essage when dep planning fails

[Hackage]

#694: mention --constraint flag in error essage when dep planning fails
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  Reporter:  duncan              |        Owner:     
      Type:  enhancement         |       Status:  new
  Priority:  normal              |    Milestone:     
 Component:  cabal-install tool  |      Version:     
  Severity:  normal              |     Keywords:     
Difficulty:  easy (<4 hours)     |   Ghcversion:     
  Platform:                      |  
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 Dependency planning problems can often be solved by running with --dry-run
 -v, seeing what the planner is selecting and forcing it to make different
 choices using the --constraint flag.

 When dependency planning fails with particular kinds of constraint
 problem, it may be useful to users to mention this. We should check if
 this advice applies to all cases where the dep planner fails, or if its
 only appropriate for certain classes of failure.

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Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/694>
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