[GHC] #13333: Typeable regression in GHC HEAD

[GHC]

#13333: Typeable regression in GHC HEAD
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        Reporter:  RyanGlScott       |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  typeable
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 simonpj):

 I think that if we have `t :: k1` and `c :: k1 ~ k2` and
 {{{
 Constraints:  [W] c :: Typeable k2 (t |> c)
 }}}
 then we can simplify the wanted constraint to
 {{{
 Bindings:    c = c1 |> Typeable ?? c
 Constraints: [W] c1 :: Typeable k1 t
 }}}
 Richard do you agree?   What is the `??`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13333#comment:6>
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