[GHC] #8576: Improve deriving error messages

[GHC]

#8576: Improve deriving error messages
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        Reporter:  nomeata                  |            Owner:  nomeata
            Type:  feature request          |           Status:  closed
        Priority:  low                      |        Milestone:
       Component:  Compiler (Type checker)  |          Version:
      Resolution:  fixed                    |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Changes (by nomeata):

 * status:  new => closed
 * resolution:   => fixed


Comment:

 After some encouragement by SPJ, I did the refactoring, introducing
 `PredOrigin` and `ThetaOrigin` for constraints where we know something
 about where they came from.

 I do ''not'' use this new type in `FunDeps` yet, as suggested. Not because
 it is not possible (see branch `wip/T8692`), but because I could not test
 that code yet (see #8592).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8576#comment:4>
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