[GHC] #15247: Use empty types for TTG extension constructors

[GHC]

#15247: Use empty types for TTG extension constructors
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        Reporter:  adamgundry        |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  TTG
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Whatever is decided, the answers should go in
 [wiki:ImplementingTreesThatGrow/TreesThatGrowGuidance]

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15247#comment:1>
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