[GHC] #14556: Core Lint error: Ill-kinded result in coercion

[GHC]

#14556: Core Lint error: Ill-kinded result in coercion
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14554            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I think this is another example of the bugs surrounding
 "flattened types have flattened kinds".  What is happening is this.
 In the RHS of `zero` we get a constraint
 {{{
     [W} (alpha :: TYPE r)  ~  TyRep X a -> (IT a |> ax_IK)
 where
     ax_IK :: IK X ~ Type   -- Just an axiom for IK
 }}}
 which should strainghtforwardly unify.  But before we do that
 we flatten `(IT a |> ax_IK)`, but alas we get just
 {{{
      (fuv :: IK X)
 where
      fuv = IT a     -- CFunEqCan
 }}}
 The cast has disappeared.  This happens in `TcFlatten`:
 {{{
 flatten_one (CastTy ty g)
 }}}
 I'm pretty sure it's just another manifestation of our existing
 set of bugs around flattening (Trac #12919).  Richard has a mostly-
 finished patch
 that cures this --- it'd be great to get it finished!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14556#comment:1>
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