[GHC] #15149: Identical distinct type family fields miscompiled

[GHC]

#15149: Identical distinct type family fields miscompiled
-------------------------------------+-------------------------------------
        Reporter:  NeilMitchell      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:  ORF
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by adamgundry):

 I think this would work, and would amount to eliminating the distinction
 between `HsRecField` (containing `FieldOcc`) and `HsRecUpdField`
 (containing `AmbiguousFieldOcc`). At the moment, the former is used for
 record construction and pattern matching (fields always resolved by the
 renamer), while the latter is used for selection and update (ambiguous
 fields resolved by the type-checker). Here "ambiguous" really means
 "ambiguous from the renamer's perspective, but not necessarily the type-
 checker".

 It would probably be simpler to defer all field resolution to the type-
 checker, though we've also considered going in the opposite direction, and
 moving all field resolution back to the renamer (see https://github.com
 /ghc-proposals/ghc-proposals/pull/84). I'm not really sure which is
 better!

 There's an awkward corner if we were to defer all field resolution to the
 type-checker, which is that `Ambiguous` fields are not currently supported
 by `DsMeta`. That is, using them inside a TH bracket will fail with a "not
 supported" error. The basic problem is that `DsMeta` works on renamed
 syntax, even though it runs after type-checking. Would that be difficult
 to change?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15149#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler