[GHC] #11309: Warn on shady data constructor export

[GHC]

#11309: Warn on shady data constructor export
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           Reporter:  dfeuer         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If a module defines a datatype and exports some, but not all, of the
 constructors, then modules importing the type will find it difficult to
 perform sensible and total case analyses. This is most likely to be a
 problem when a module that names exported constructors adds a new
 constructor. Could we get a warning for this, please?

 Note: this applies also to data family instances.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11309>
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