[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:  infoneeded
        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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Comment (by simonpj):

 The real issue is this: what error message do we *want* from the erroneous
 program in the Description?

 There really is an error:
  * the generic default method is used in the `instance` because no
 explicit method for `reflexive` is given.
  * but the generic default requires `(Eq a)` and that is not available.

 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?

 The difficulty is that for generic defaults, for the class decl we
 generate
 {{{
 $gdmreflexive :: (C a, Eq a) => a -> Bool
 $gdmreflexive x = x==x
 }}}
 This part is fine.

 For the missing method binding in the `instance` we generate
 we generate ''source code'' looking like
 {{{
   reflexive = $gdmreflexive
 }}}
 Now we typecheck that, which gives the error message.  And you can see it
 might be hard to generate the "right" error message.

 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?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10087#comment:8>
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