[GHC] #12814: Should GND infer an instance context when deriving method-free classes?

[GHC]

#12814: Should GND infer an instance context when deriving method-free classes?
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11369, #12810    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Replying to [comment:2 goldfire]:
 > Just infer the class constraint based on the constraints that arise when
 type-checking the generated definition.

 I agree with this; and it's precisely what Ryan's patch does.
 `mkNewTypeEqn` generates the constraints that are necessary to satisfy the
 instance decl; they are then simplified, perhaps to nothing at all.  We
 are going to generate
 {{{
 instance ?? => C (N a) where
   ...type instances...
   op = coerce (op :: rep-ty)
 }}}
 So for each method (if any) we need

 * `C rep_ty` so that we have the underlying `op`
 * `op_ty[rep_ty] ~R op_ty[N a]` to for the coercion of the `op`

 If there are no methods we don't need any of those constraints.  Ryan's
 notes above show that the result may perhaps not be what you want, but if
 not, just use a standalone deriving and specify the instance context.

 So I vote for this patch!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12814#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler