[GHC] #13626: Ambiguity when exporting a data constructor despite qualification

28 Apr 2017

      #13626: Ambiguity when exporting a data constructor despite qualification
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           Reporter:  Lemming        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
  (Parser)                           |
           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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 {{{
 $ cat A.hs
 module A where

 data T = Cons

 $ cat B.hs
 module B (T(T)) where

 import qualified A

 data T = T

 $ ghc-8.2.0.20170422 -Wall B.hs
 [2 of 2] Compiling B                ( B.hs, B.o ) [A changed]

 B.hs:1:11: error:
     • Ambiguous occurrence ‘T’
       It could refer to either ‘A.T’,
                                imported qualified from ‘A’ at B.hs:3:1-18
                             or ‘T’, defined at B.hs:5:1
     • In the export: T(T)
   |
 1 | module B (T(T)) where
   |           ^^^^
 }}}

 There should be no ambiguity, because there is only one data constructor
 named `T`.
 Even if `A.T`'s data constructor would also be called `T`
 there would be no ambiguity because `A` is imported with qualification.
 The problem does not appear, if `B.T`'s data constructor is named `Cons`.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13626
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