[GHC] #9422: EPT caching on --make can make spurious instances visible

[GHC]

#9422: EPT caching on --make can make spurious instances visible
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              Reporter:  ezyang      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  accepts invalid program            |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by ezyang):

 In our most recent Backpack information, SPJ described a nice and simple
 way of solving this problem.  Continue to maintain a global EPT, but with
 every type class instance, record the module it originated from. Now, when
 we do a lookup into the EPT table, we need to check if that module is
 transitively reachable from the current module; if it is not, then it is
 discarded. The transitivity check can be done lazily, so, hopefully, when
 this bad behavior doesn't occur, we can avoid doing all of the work of
 slurping in all the imports to figure out what type instances are
 available and which are not.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9422#comment:1>
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