[GHC] #14992: Allow existentials to UNPACK

[GHC]

#14992: Allow existentials to UNPACK
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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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 Consider
 {{{
 data T a where
   MkT :: b -> (b->a) -> T a

 data Wuggle where
   MkW :: {-# UNPACK #-} !T Int -> Wuggle
 }}}
 Here `MkT` has an existential, but there is really no reason that it can't
 unpack into the data constructor for `MkW`.  But it is currently rejected.

 Fixing this would require quite a bit of new plumbing.  E.g. the
 existential type variable of the worker and wrapper would differ, which is
 not true today.  But morally it's the Right Thing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14992>
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