[GHC] #13546: Kind error with type equality

[GHC]

#13546: Kind error with type equality
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        Reporter:  int-index         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by int-index):

 Ok, so if I write

 {{{#!hs
 type family UnT (a :: Type) :: k where
   UnT (T (t :: k')) = t
 }}}

 the `k ~ k'` proof for the RHS is provided by the LHS.

 Can I write a LHS that will match on `t :: k'` for all kinds `k'` and try
 to unify `k ~ k'` only on the RHS?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13546#comment:5>
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