[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 simonpj):

 If I remember right, the export list itself does allow extra commas.   So
 it seems a bit inconsistent not to allow the same story for the `Foo( P1,
 P2 )` form.

 So this would be ok with me.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12389#comment:9>
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