[GHC] #15706: Refactor NewHsTypeX to DerivedCoreTy

[GHC]

#15706: Refactor NewHsTypeX to DerivedCoreTy
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           Reporter:  int-index      |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  low            |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.6.1
           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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 `NewHsTypeX` is not a good name for this construct:

 {{{
 data NewHsTypeX
   = NHsCoreTy Type -- An escape hatch for tunnelling a *closed*
                    -- Core Type through HsSyn.
 }}}

 * the `New` prefix refers to the fact that it is a new approach, rather
 than to a property of the entity
 * it is not suggestive of the way `NewHsTypeX` is used in the code and why
 it exists

 A better name would be `DerivedCoreTy`. The reason for `NHsCoreTy`
 existence and its invariant of being closed is the `deriving` mechanism.

 Doing this renaming also exposes incorrect uses of `NHsCoreTy` in a couple
 of places that violate the invariant (in some calls to `unifyKind`).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15706>
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