[GHC] #10087: DefaultSignatures: error message mentions internal name

[GHC]

#10087: DefaultSignatures: error message mentions internal name
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        Reporter:  andreas.abel      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.4
  checker)                           |
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  infoneeded => new


Comment:

 Thanks, Simon!
 [https://ghc.haskell.org/trac/ghc/ticket/10087?replyto=8#comment:7 This
 comment] helps tremendously in figuring out how things currently work.

 Replying to [comment:8 simonpj]:
 > So we need something like
 > {{{
 >     No instance for (Eq D) arising from
 >        the generic default method for `reflexive`
 >     In the instance declaration for ‘C D’
 > }}}
 > Would that be about right?

 I agree completely! I was confused because after reading
 [https://ghc.haskell.org/trac/ghc/ticket/10087?replyto=8#comment:1 Pedros'
 comment], I was under the impression he was implying that code should be
 legal. I probably misinterpreted it wildly and came to a very wrong
 conclusion.

 > Better perhaps to do what happens for non-generic default methods, which
 is to generate typechecked code directly (and emit some constraints).
 Compare what we do for the `Nothing` case of `DefMethInfo` in `tc_default`
 in `tcMethods` in `TcInstDcls`.
 >
 > Does that make sense?

 That seems sensible. Clearly, the mechanism we use for `VanillaDM` works
 well, because I've never seen an error message mention a name that begins
 with `$dm` :) If we modify the code that handles `GenericDM` to use the
 same tricks, that would probably make the error message much more
 palatable. We could also insert the phrase `"the generic default method
 for"` to make the origin of the issue clearer.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10087#comment:9>
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