[GHC] #12088: Promote data family instance constructors

[GHC]

#12088: Promote data family instance constructors
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        Reporter:  alexvieth         |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11348            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alexvieth):

 > Maybe someone can offer a patch?

 [https://phabricator.haskell.org/D2272 Here's my attempt.]

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12088#comment:3>
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