[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 (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by bgamari:

@@ -4,2 +4,2 @@
- renamed), renaming them causes `bkpreex06` to fail with the panic
- `find_lbl missing foo`.
+ renamed), renaming them (in `RnModIface`) causes `bkpreex06` to fail with
+ the panic `find_lbl missing foo`.

New description:

 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 (in `RnModIface`) 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#comment:1>
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