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

[GHC]

#13458: Panic with unsafeCoerce and -dcore-lint
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE MagicHash, TypeInType, ScopedTypeVariables #-}
 module UIOST where
 import GHC.Exts
 import Data.Kind
 import Unsafe.Coerce

 unsafeCoerce' :: forall (r :: RuntimeRep)
                        (a :: TYPE r) (b :: TYPE r).
                 a -> b
 unsafeCoerce' = unsafeCoerce unsafeCoerce
 }}}

 Compiling this with optimization and `-dcore-lint` produces a panic:

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20170308 for x86_64-unknown-linux):
         runtimeRepPrimRep
   typePrimRep (a_aW7 :: TYPE r_aW6)
   r_aW6
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1191:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in
 ghc:Outputable
         pprPanic, called at compiler/simplStg/RepType.hs:360:5 in
 ghc:RepType
 }}}

 Using `unsafeCoerce# unsafeCoerce` produces the panic without
 optimization. I have not been able to reproduce the problem without
 `-dcore-lint`.

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