[GHC] #12615: Record pattern synonyms cause spurious name shadowing warnings

[GHC]

#12615: Record pattern synonyms cause spurious name shadowing warnings
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        Reporter:  gelisam           |                Owner:  mpickering
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
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 mpickering):

 This isn't actually a problem with record pattern synonyms per se but
 something that is exposed by them.

 You can also trigger the problem with

 {{{#!hs
 {-# LANGUAGE NoImplicitPrelude, PatternSynonyms #-}
 {-# OPTIONS_GHC -Wall #-}
 module Test where

 x = ()

 pattern Point2 :: () -> () -> ((), ())
 pattern Point2 x y = (x, y)
 }}}

 This is because the way checking for shadowed names works assumes that all
 patterns are introducing binders. Clearly in the case of pattern synonyms
 this assumption fails so we need to refine this check.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12615#comment:2>
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