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

[GHC]

#10087: DefaultSignatures: error message mentions internal name
-------------------------------------+-------------------------------------
        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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Changes (by RyanGlScott):

 * status:  new => infoneeded


Comment:

 I'm a bit confused about the state of affairs for this ticket.

 Pedro, I don't really understand what you mean when you say "So perhaps
 that's what we should do in this case, too." After all, this code:

 {{{#!hs
 {-# LANGUAGE DefaultSignatures #-}

 class C a where
   reflexive :: a -> Bool
   default reflexive :: Eq a => a -> Bool
   reflexive x = x == x

 data D

 instance C D where
 }}}

 and this code:

 {{{#!hs
 class C a where
   reflexive :: Eq a => a -> Bool
   reflexive x = x == x

 data D

 instance C D
 }}}

 appear to be fundamentally different. In the former, `reflexive` defines a
 function that //requires// an `Eq a` constraint when the user doesn't
 implement it. In the latter, `reflexive` defines a function that
 //presupposes// that `a` is an instance of `Eq`. When viewed in this
 light, shouldn't the former code error and the latter code be OK?

 Simon, what are `GenDefMeth` and `NoDefMeth`? I can't find anything in the
 source about them (save for one possibly outdated comment on `NoDefMeth`).
 Were they replaced by `DefMethSpec` (i.e,
 [http://git.haskell.org/ghc.git/blob/93d85af9fec968b43452891ec7b10382a4a99a38:/compiler/basicTypes/BasicTypes.hs#l774
 this]) at some point? If so, are your comments about a proposed
 refactoring still relevant?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10087#comment:6>
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