[GHC] #14561: Panic on levity polymorphic very unsafe coerce

[GHC]

#14561: Panic on levity polymorphic very unsafe coerce
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I strongly prefer the latter. It makes me nervous not to have a true back-
 door to the type system (which we would lose if we dropped the levity-
 polymorphism from `unsafeCoerce#`).

 It should be easy enough to check for saturation in the desugarer.

 But, actually, there's a third option:
  * Allow silly uses of `unsafeCoerce#` to cause a panic.

 The user is clearly taking on the risk of a runtime crash with
 `unsafeCoerce#`. We could just say they also risk taking on a compile-time
 crash. In this case, I think the fix (the saturation check) is easy
 enough, but I don't feel strongly committed to eradicating a hard-to-fix
 panic if the user abuses the type system with such a low-level operation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14561#comment:2>
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