[GHC] #10000: Derived DataKinds Typeable instance collision leads to SafeHaskell violation

[GHC]

#10000: Derived DataKinds Typeable instance collision leads to SafeHaskell
violation
-------------------------------------+-------------------------------------
              Reporter:  shachaf     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
 The derived `Typeable` instances for a kind (with `DataKinds`) is equal to
 the
 instance of a type with the same name:

 {{{
 λ> data T = T
 λ> typeRep (Proxy :: Proxy T) == typeRep (Proxy :: Proxy 'T)
 True
 }}}

 This can be used to violate `SafeHaskell` easily:

 {{{#!hs
 {-# LANGUAGE AutoDeriveTypeable, DataKinds, TypeFamilies, RankNTypes #-}

 module Unsafe where

 import Data.Typeable

 data T = T

 data family F p

 newtype instance F (Proxy  T) = ID (forall a. a -> a)
 newtype instance F (Proxy 'T) = UC (forall a b. a -> b)

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

 {{{
 $ ghc -XSafe Unsafe.hs -e "uc 'a' :: Int"
 97
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10000>
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