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

[GHC]

#13458: Panic with unsafeCoerce and -dcore-lint
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by dfeuer:

Old description:

> {{{#!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`.

New description:

 {{{#!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 id
 }}}

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

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13458#comment:1>
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