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

[GHC]

#13458: Panic with unsafeCoerce and -dcore-lint
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        Reporter:  dfeuer            |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:
      Resolution:                    |  LevityPolymorphism
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:                    |
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Comment (by simonpj):

 Another thought. The current `unsafeCoerce#` (defined in `MkId`) has type
 {{{
     -- unsafeCoerce# :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep)
     --                         (a :: TYPE r1) (b :: TYPE r2).
     --                         a -> b
 }}}`
 But it's super-dangerous to coerce from (say) a pointer type to a `Word#`
 and back.  Perhaps the slightly more restrictive type
 {{{
     -- unsafeCoerce# :: forall (r :: RuntimeRep)
     --                         (a :: TYPE r) (b :: TYPE r).
     --                         a -> b
 }}}
 would correctly capture many the "rule" specified in the documentation for
 `unsafeCoerce`?

 Regardless, a crash is bad.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13458#comment:10>
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