[GHC] #12389: Limit duplicate export warnings for datatypes

[GHC]

#12389: Limit duplicate export warnings for datatypes
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11959            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Replying to [comment:5 rwbarton]:
 > Well you don't really want to "silence the warning from duplicate
 constructors/patterns within the list". You want to silence ''this''
 duplicate of `Bar`. If you happen to repeat a different constructor
 elsewhere in the export list by accident, you'd still want a warning about
 that. So it would more appropriate to use a mechanism for locally
 disabling warnings.

 Local suppression is very heavy for a warning about something as
 relatively unimportant as this. I'd be happy with a heuristic: accidental
 duplicates are typically less likely in a constructor/pattern export list
 than in a (typically much longer) module export list.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12389#comment:6>
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