[GHC] #9788: `coerce` has an impossible type

[GHC]

#9788: `coerce` has an impossible type
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       Reporter:  goldfire           |                   Owner:  goldfire
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 {{{
 GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Prelude> :set -fprint-explicit-foralls
 Prelude> :set -fprint-explicit-kinds
 Prelude> import Data.Coerce
 Prelude Data.Coerce> :t coerce
 coerce
   :: forall (k :: BOX) (a :: k) (b :: k). Coercible k a b => a -> b
 }}}

 Are those types `a` and `b` really of kind `k`? They certainly shouldn't
 be.

 This is worse in HEAD: GHC panics. (Actually, I think panicking is an
 improvement.)

 Easy to fix, happily. Will do so in the course of other work.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9788>
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