[GHC] #13102: orphan family instances can leak through the EPS in --make mode

[GHC]

#13102: orphan family instances can leak through the EPS in --make mode
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        Reporter:  rwbarton          |                Owner:  rwbarton
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 * I think there are two possible cases where code change would be
 necessary (but I don't think any of them actually apply):
    - If GHC.Exts defined orphans, we should still treat those orphans as
 visible. But I don't think we have any orphans. Would be good to check.
    - You are right that one can't define an instance of IsList without
 importing it, so we should be able to get away without the check. And
 evidently this doesn't apply if GHC.Exts does have orphans, because that
 means we can define an instance without importing GHC.Exts
 * Yes, I guess we have to handle known-key things. Hmm, we don't call
 `checkWiredInTyCon` for those, so there might be trouble.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13102#comment:17>
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