[GHC] #15282: Document how equality-bearing constructors are promoted in Core

[GHC]

#15282: Document how equality-bearing constructors are promoted in Core
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #14845
Differential Rev(s):                 |         Wiki Page:
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 In Phab:D4728, Simon was utterly baffled as to how one could promote the
 `MkT` constructor in:

 {{{#!hs
 data T a where
   MkT :: (a ~ Int) => T a
 }}}

 Richard knows the inner machinations of how this works (including what
 coercions are used in the Core that `'MkT` desugars to), but not many
 others do. Simon requested that Richard document this knowledge in a Note
 somewhere. This ticket exists to keep track of this request.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15282>
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