[GHC] #8576: Improve deriving error messages

[GHC]

#8576: Improve deriving error messages
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        Reporter:  nomeata                  |            Owner:  nomeata
            Type:  feature request          |           Status:  new
        Priority:  low                      |        Milestone:
       Component:  Compiler (Type checker)  |          Version:
      Resolution:                           |         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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Comment (by nomeata):

 Theoretically possible, but would require a decent amount of code
 reshuffling: The `CtOrigin` is currently created in `derivTyData` (and
 others), and passed down to `mkEqnHelp` to `mkDataTypeEqn` to
 `mk_data_eqn`, where the `inferred_constraints`, which are obtained from
 `inferConstraints` as a list, put together with the `CtOrigin` and put
 into one `EarlyDerivSpec`, which does not support different `CtOrigin`s
 for different constraints.

 I do like fancy error messages, but the code (passing around plain
 `[Type]` for the constraint) has some elegance. Is this feature worth
 uprooting that?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8576#comment:1>
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