[GHC] #14892: Field imposters with DuplicateRecordFields

5 Mar 2018

      #14892: Field imposters with DuplicateRecordFields
-------------------------------------+-------------------------------------
           Reporter:  philderbeast   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  MacOS X
       Architecture:  x86_64         |   Type of failure:  GHC accepts
  (amd64)                            |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  13644
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 There's a [[https://github.com/BlockScope/ghc-panic-12158|test case]] for
 this ghc panic. If I use `NamedFieldPuns` I wish this could more
 delicately pick the right field. If my record use is qualified then please
 restrict the set of candidate field names to use punned.

 {{{
 {-# LANGUAGE DuplicateRecordFields #-}
 module Geodesy (X(..), Y(..)) where

 data X a = X {x :: a}
 data Y a = Y {x :: a}
 }}}

 {{{
 {-# LANGUAGE NamedFieldPuns #-}
 module GhcPanic12158 where

 import qualified Geodesy as G (X(..))
 import Geodesy (Y(..))

 update :: G.X a -> G.X a
 update G.X{x} = G.X{x = x}
 }}}

 {{{
...
stack build
 ghc-panic-translateConPatVec-lookup-0.1.0: build (lib)
 Preprocessing library for ghc-panic-translateConPatVec-lookup-0.1.0..
 Building library for ghc-panic-translateConPatVec-lookup-0.1.0..
 [1 of 3] Compiling Geodesy
 [2 of 3] Compiling GhcPanic12158
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-apple-darwin):
         translateConPatVec: lookup
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 There are some fixes;

 1. Add DuplicateRecordFields.

 {{{
 {-# LANGUAGE NamedFieldPuns #-}
 {-# LANGUAGE DuplicateRecordFields #-}
 module GhcPanic12158 where

 import qualified Geodesy as G (X(..))
 import Geodesy (Y(..))

 update :: G.X a -> G.X a
 update G.X{x} = G.X{x = x}
 }}}

 2. Use qualified field names.

 {{{
 {-# LANGUAGE NamedFieldPuns #-}
 module GhcPanic12158 where

 import qualified Geodesy as G (X(..))
 import Geodesy (Y(..))

 update :: G.X a -> G.X a
 update G.X{G.x} = G.X{G.x = x}
 }}}

 Interestingly, if I don't import the record with the clashing field name
 then GHC complains.

 {{{
 {-# LANGUAGE NamedFieldPuns #-}
 module GhcPanic12158 where

 import qualified Geodesy as G (X(..))

 update :: G.X a -> G.X a
 update G.X{x} = G.X{x = x}
 }}}

 {{{
...
stack build
 ghc-panic-translateConPatVec-lookup-0.1.0: unregistering
 ghc-panic-translateConPatVec-lookup-0.1.0: build (lib)
 Preprocessing library for ghc-panic-translateConPatVec-lookup-0.1.0..
 Building library for ghc-panic-translateConPatVec-lookup-0.1.0..
 [3 of 3] Compiling GhcPanic12158
/ghc-panic-12158/earth/library/GhcPanic12158.hs:8:12: error: Not in scope:
 ‘x’
   |
 8 | update G.X{x} = G.X{x = x}
   |            ^

 /ghc-panic-12158/earth/library/GhcPanic12158.hs:8:21: error: Not in scope:
 ‘x’
   |
 8 | update G.X{x} = G.X{x = x}
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14892
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