[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:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In --make mode there is a single EPS whose `eps_fam_inst_env` holds all
 type family instances that have been read from interface files from
 external packages for any reason. The type checker uses this field
 pervasively via `tcGetFamInstEnvs`.

 When compiling multiple files it's fairly easy to set up a situation in
 which

 1. the first module to be compiled `A` loads an interface file `O.hi` from
 another package containing an orphan family instance (say, because it
 imports the module directly);

 2. a later module `B` uses this instance to reduce a type, even though `B`
 has no dependency at all on the module `O` defining the instance.

 (The only tricky bit in arranging this is that since `B` cannot depend on
 `A`, a little good fortune is needed for GHC to decide to compile `B`
 after `A`.)

 Now another module could import `B` and obtain a function whose definition
 relies on a type family instance that is not visible from `B` at all,
 compromising type safety.

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