[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 adamgundry):

 Ouch! I conjecture that the following lighter restriction will restore
 type soundness: a hs-boot file must list all type instance declarations
 used (transitively) in the typechecking of any definition whose
 declaration appears in the hs-boot file. (That is, a type instance
 declaration must be listed if its axiom would appear in the fully-unfolded
 elaborated Core term corresponding to any declaration in the hs-boot
 file.)

 Even if this conjecture is correct, of course, this may be unreasonably
 hard to implement/explain.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562#comment:2>
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