[GHC] #14747: DisambiguateRecordFields fails for PatternSynonyms

31 Jan 2018

      #14747: DisambiguateRecordFields fails for PatternSynonyms
-------------------------------------+-------------------------------------
           Reporter:  adamgundry     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11283
Differential Rev(s):                 |         Wiki Page:
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 Consider:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}

 module A where

 pattern S{x} = [x]
 }}}

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, DisambiguateRecordFields #-}

 module B where

 import A

 pattern T{x} = [x]

 e = S { x = 42 }
 }}}

 Compiling module B fails with `Ambiguous occurrence ‘x’` in the definition
 of `e`. In principle, `DisambiguateRecordFields` should select the field
 belonging to the `S` "data constructor". However, the current
 implementation of this works by identifying the parent type constructor,
 which doesn't exist for a pattern synonym.

 This continues to fail if `T` is replaced by a data type with a field `x`.
 If `S` is replaced by a data type, however, it starts working.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14747
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