[GHC] #13149: Giving Backpack a Promotion

[GHC]

#13149: Giving Backpack a Promotion
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        Reporter:  ezyang            |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  Research
       Component:  Compiler (Type    |  needed
  checker)                           |              Version:  8.1
      Resolution:                    |             Keywords:  backpack
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 goldfire):

 Yes, with `-XTypeInType`, coercions appear in types. For example:

 {{{
 type family F a where
   F Bool = Type

 foo :: forall (a :: F Bool). a -> a
 foo x = x
 }}}

 (Panics in GHC 8.0.1, but works in HEAD.)

 The type of `foo` will contain a reference to a coercion axiom.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13149#comment:1>
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