[GHC] #9562: Type families + hs-boot files = unsafeCoerce

[GHC]

#9562: Type families + hs-boot files = unsafeCoerce
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        Reporter:  goldfire          |                Owner:  ezyang
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  7.8.3
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  SafeHaskell hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10270            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 rwbarton's comment:16 looks very plausible. I can't quite convince myself
 that it's right, but I also can't think of a counterexample. It's
 certainly easier to explain (and likely to implement) than many other
 solutions mentioned here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562#comment:17>
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