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

[GHC]

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

 Richard Eisenberg does it again! Ticket #9422 seems related. I support the
 proposed solution. (Note that the situation is different for module
 signatures ala #9252, because when we compile against a signature's hi
 file we can pull in the original implementation to get its type family
 instances for a consistency check.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562#comment:1>
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