[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):

 simonpj and I chatted about this, and we came up with a short term and a
 long term solution.

 * The short term solution is to special-case GHC.Exts so that it is always
 visible. This should be pretty easy to do.

 * But, if we look at how the compiler handles finding instances of wired
 in things, there is a `checkWiredInTyCon` function which we specifically
 call in order to bring in the instances for a wired in thing. So really,
 the desired semantics are, if we call `checkWiredInTyCon` on a `TyCon`, we
 want to act AS IF we had an implicit import of this `TyCon`. This means
 checking it for family instance consistency (see #13251) and considering
 it visible. But now that I think about it, it seems very awkward to
 actually get this info to the list of visible info, since we won't really
 bang on the TyCon until we're about to do an instance lookup; might be a
 little fiddly to get that to work. But the long term solution is something
 like this: an implicit reference to a wired in thing counts as an import
 to that defining module.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13102#comment:15>
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