[GHC] #15814: Orphan Instance Overlap Error Message

[GHC]

#15814: Orphan Instance Overlap Error Message
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        Reporter:  parsonsmatt       |                Owner:  (none)
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5377
       Wiki Page:                    |
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Comment (by dminuoso):

 `- an orphan module we can report how $df came to be in scope.`
 You need a transitive closure of depended upon orphan modules and finally
 some `[ImportedBy]` for each such module. We cannot recover that
 information from TcGblEnv currently.

 My code already takes your suggested second approach, but it can't do a
 full job because the record `mi_deps :: Dependencies` has no field
 containing the full transitive closure of depended upon modules. It only
 contains that information for orphan modules.

 I hope the changes I pushed make my code a bit clearer.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15814#comment:5>
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