[Hackage] #400: fail and warnings for build configurations

[Hackage]

#400: fail and warnings for build configurations
----------------------------+-----------------------------------------------
  Reporter:  duncan         |        Owner:       
      Type:  enhancement    |       Status:  new  
  Priority:  normal         |    Milestone:       
 Component:  Cabal library  |      Version:       
  Severity:  normal         |     Keywords:       
Difficulty:  normal         |   Ghcversion:  6.8.3
  Platform:                 |  
----------------------------+-----------------------------------------------
 People expect `buildable: False` to fail the current configuration (rather
 than letting it configure and then not working at build time). But perhaps
 we should make that more explicit and allow a message, eg instead of:

 {{{
 if os(windows)
   buildable: False
 }}}
 allow
 {{{
 if os(windows)
   fail: it doesn't work on windows because, blah blah blah
 }}}

 And similarly we might want to warn users about selecting particular
 configurations eg:

 {{{
 if flag(http)
   ...
 else
   warn: no http implementation selected, only local
         operations will be possible. You probably really
         want some http implementation.
 }}}

 The semantics roughly is that it's a workable but sub-optimal
 configuration.

 The solver should try to avoid warnings, by selecting another alternative.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/400>
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