[GHC] #12548: Exported pattern synonyms does not mark top-level bindings in RHS as used

[GHC]

#12548: Exported pattern synonyms does not mark top-level bindings in RHS as used
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           Reporter:  pkmx           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}

 module Foo (pattern P) where

 x :: Int
 x = 0

 pattern P :: Int
 pattern P <- _ where
         P = x
 }}}

 gives:

 {{{
 Foo.hs:6:1: warning: [-Wunused-top-binds]
     Defined but not used: ‘x’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12548>
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