[GHC] #9702: Offer a weaker name shadowing warning

[GHC]

#9702: Offer a weaker name shadowing warning
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:  warning, module    |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 Suppose I have a module

 {{{#!hs
 module Survive where
 import HuntGather

 flounder = goFishing river
   where
     goFishing = ...
 }}}

 If the maintainer of `HuntGather` decides to add a `goFishing` function,
 my module will suddenly get a name shadowing warning, although there is
 little risk of confusion. So I think it makes a lot of sense to offer, at
 least, a version that ignores shadowing of *implicitly imported* names,
 and to use this for `-Wall` instead of the current version. Another step
 down would be to ignore shadowing of all top-level names, but having three
 levels of name shadowing warning would be too much trouble by far.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9702>
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