[GHC] #13750: GHC produces incorrect coercions in hairy code

[GHC]

#13750: GHC produces incorrect coercions in hairy code
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.2
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13429            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I'm no "type guru", but I find this development quite unsurprising. The
 "utterly bogus definition" that you pointed out in the original comment
 concerns the `All` type class, which has `AllF` as a superclass. The nub
 of #13429 concerns improper specialization of dictionary functions with
 superclasses, and `All` falls into the category. Combine incorrect
 specialization with `unsafeCoerce`, and segfaults are just around the
 corner.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13750#comment:11>
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