[GHC] #16310: Program fails with "Impossible case alternative" when optimized

[GHC]

#16310: Program fails with "Impossible case alternative" when optimized
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:  wontfix           |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:6 goldfire]:
 > But perhaps it would be better to put the use of `unsafeCoerce` in
 `funExt` directly. I wonder if that quells the problem.

 Alas, defining `funExt _ = unsafeCoerce (Refl @f)` does no good—it still
 produces `Impossible case alternative` when ran.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16310#comment:7>
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