[GHC] #12699: Suspicious treatment of renaming of field labels

[GHC]

#12699: Suspicious treatment of renaming of field labels
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           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While working on my rework of interface file names (Phab:D2467) I noticed
 that there is some funniness in the treatment of renaming field selectors.
 Namely, `ifaceConDeclFields` are `IfaceTopBndr`s (and should therefore be
 renamed), renaming them causes `bkpreex06` to fail with the panic
 `find_lbl missing foo`.
 Looking at the environment reveals that there are selectors named `foo` in
 scope, but they were not renamed.

 I believe this is due to the implementation of `tcIfaceDataCons`, which
 typechecks the fields with `mapM (traverse lookupIfaceTop)
 (ifaceConDeclFields if_cons)`. In essence, this projects the field labels
 back to `OccName`s (in `ifaceConDeclFields`) and then looks each of these
 `OccName`s up in the name cache, thereby retrieving the un-renamed `Name`.
 This seems wrong.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12699>
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