[GHC] #15470: Record projections with ambiguous types

[GHC]

#15470: Record projections with ambiguous types
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        Reporter:  sweirich          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      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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Comment (by simonpj):

 I forgot to put in the diff
 {{{
 modified   compiler/typecheck/TcTyDecls.hs
 @@ -893,7 +893,7 @@ mkOneRecordSelector all_cons idDetails fl
               | otherwise =  map mk_match cons_w_field ++ deflt
      mk_match con = mkSimpleMatch (mkPrefixFunRhs sel_lname)
                                   [L loc (mk_sel_pat con)]
 -                                 (L loc (HsVar noExt (L loc field_var)))
 +                                 match_body
      mk_sel_pat con = ConPatIn (L loc (getName con)) (RecCon rec_fields)
      rec_fields = HsRecFields { rec_flds = [rec_field], rec_dotdot =
 Nothing }
      rec_field  = noLoc (HsRecField
 @@ -905,6 +905,19 @@ mkOneRecordSelector all_cons idDetails fl
      sel_lname = L loc sel_name
      field_var = mkInternalName (mkBuiltinUnique 1) (getOccName sel_name)
 loc

 +    match_body = L loc (HsVar noExt (L loc field_var))
 +{-
 +    match_body = foldl app_tv (L loc (HsVar noExt (L loc field_var)))
 field_tvs
 +
 +    app_tv :: LHsExpr GhcRn -> TyVar -> LHsExpr GhcRn
 +    app_tv hs_fun tv = L loc (HsAppType (mk_tv_ty tv) hs_fun)
 +
 +    mk_tv_ty :: TyVar -> LHsWcType GhcRn
 +    mk_tv_ty tv = mkEmptyWildCardBndrs $ L loc $
 +                  HsTyVar noExt NotPromoted $
 +                  L loc (getName tv)
 +-}
 }}}
 If you try commenting out the definition of `match_body` and replacing it
 with the one that is commented out in this diff, you get my attempt.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15470#comment:3>
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