[GHC] #11216: RebindableSyntax with pattern guards needs `fail`

[GHC]

#11216: RebindableSyntax with pattern guards needs `fail`
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           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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 If I say

 {{{
 {-# LANGUAGE RebindableSyntax #-}

 module Bug where

 foo :: (a, b) -> ()
 foo x | (_,_) <- x = ()
 }}}

 I get

 {{{
 Bug.hs:6:9: error: Not in scope: ‘fail’
 }}}

 Yet even if I bring `fail` into scope, it's not mentioned in the Core
 produced by my program. And it really shouldn't: there's not a monad in
 sight!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11216>
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