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

[GHC]

#15149: Identical distinct type family fields miscompiled
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        Reporter:  NeilMitchell      |                Owner:  adamgundry
            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:  #14747            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by adamgundry):

 I think I've come across a simpler way to fix this and #14747. At the
 moment, `rnHsRecFields` goes to some trouble to figure out the parent type
 constructor of the data constructor (in `find_tycon`) and then does field
 name lookup using the parent type constructor. But we already have
 `lookupConstructorFields` which lets us directly find out the
 `FieldLabel`s of a data constructor! (This is used for expanding dot-dot
 patterns.)

 So why don't we just use `lookupConstructorFields` and search amongst them
 for the right one? We'd need to be a bit careful to still check the name
 is in scope (with the right module qualifier, if any), but that should be
 simple if we know the unambiguous selector name already.

 Moving field label resolution to the typechecker still might be worth
 doing, because it should get rid of quite a bit of duplication. But it's
 not a small task (e.g. because of dot-dot patterns) so I think it's worth
 pursuing the small fix first.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15149#comment:9>
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