[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 simonpj):

 Thanks for working on this.

 Indeed, a careful `Note` to explain how it works, and some more type
 signatures on local definitions, would be helpful.  I could not understand
 the code.

 I think your idea is something like this, supposing we are compiling
 module M, and are reporting an instance whose dictionary function is
 `$df`.

 * Given a `$df` we know the module D in which it is defined, via
 `nameModule`.
 * If that module D is
   * M itself, or
   * one of the module that M directly imports
   * an orphan module
   we can report how `$df` came to be in scope.
 * Those three cases are easy because we have that info to hand in the
 `TcGblEnv`.

 You code looks a bit complicated if I'm right, but maybe I'm missing
 something.

 I also wonder about doing the full job, when `$df` is in scope indirectly.
 After all, that's the situation in which the user is going to be most
 puzzled!  What we want is to ask
 * For each `import B`, does `B` transitively import `D`?
 And that info is also conveniently available, in the `mi_deps` field of
 each `ModIface`.  So it should be easy to do to full job, no exceptions.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15814#comment:4>
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