[GHC] #13458: Panic with unsafeCoerce and -dcore-lint

[GHC]

#13458: Panic with unsafeCoerce and -dcore-lint
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:
      Resolution:                    |  LevityPolymorphism
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  typecheck/should_compile/T13458
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by rwbarton):

 This is causing a large number of warnings to be printed while building
 GHC, of the form
 {{{

 <no location info>: warning:
     In the expression: byteArrayContents#
                          (ipv3_s9ST
                           `cast` (UnsafeCo representational
                                     (MutableByteArray# RealWorld)
 ByteArray#
                                   :: (MutableByteArray# RealWorld :: TYPE
 'UnliftedRep)
                                      ~R#
                                      (ByteArray# :: TYPE 'UnliftedRep)))
     Unsafe coercion: left-hand type is levity-polymorphic
       From: MutableByteArray# RealWorld
         To: ByteArray#

 <no location info>: warning:
     In the expression: byteArrayContents#
                          (ipv3_s9ST
                           `cast` (UnsafeCo representational
                                     (MutableByteArray# RealWorld)
 ByteArray#
                                   :: (MutableByteArray# RealWorld :: TYPE
 'UnliftedRep)
                                      ~R#
                                      (ByteArray# :: TYPE 'UnliftedRep)))
     Unsafe coercion: right-hand type is levity-polymorphic
       From: MutableByteArray# RealWorld
         To: ByteArray#
 }}}
 and these statements do not appear to be correct.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13458#comment:14>
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