[GHC] #9858: Typeable instances should be kind-aware

[GHC]

#9858: Typeable instances should be kind-aware
-------------------------------------+-------------------------------------
        Reporter:  dreixel           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D652
-------------------------------------+-------------------------------------
Changes (by oerjan):

 * priority:  normal => highest


Comment:

 Sorry, but GHC 7.10.1 is still vulnerable.

 {{{
 -- This exploit still works in GHC 7.10.1.
 -- By Shachaf Ben-Kiki, Ørjan Johansen and Nathan van Doorn

 {-# LANGUAGE Safe #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE ImpredicativeTypes #-}

 import Data.Typeable

 type E = (:~:)
 type PX = Proxy (((),()) => ())
 type PY = Proxy (() -> () -> ())

 data family F p a b

 newtype instance F a b PX = ID (a -> a)
 newtype instance F a b PY = UC (a -> b)

 {-# NOINLINE ecast #-}
 ecast :: E p q -> f p -> f q
 ecast Refl = id

 supercast :: F a b PX -> F a b PY
 supercast = case cast e of
     Just e' -> ecast e'
   where
     e = Refl
     e :: E PX PX

 uc :: a -> b
 uc = case supercast (ID id) of UC f -> f
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:76>
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