[GHC] #14130: MultiWayIf parse behavior changed in 8.2.1

16 Aug 2017

      #14130: MultiWayIf parse behavior changed in 8.2.1
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.2.1
  (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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 I just noticed while working on Jenkins that
 ed7a830de6a2ea74dd6bb81f8ec55b9fe0b52f28 fails to build with GHC 8.0.1
 with a parse error, yet succeeds with 8.2.1.

 The issue is demonstrated by the following program,
 {{{#!hs
 {-# LANGUAGE MultiWayIf #-}
 main =
     if | True
        -> putStrLn "hasdf"
 }}}
 Under GHC 8.0.1 this fails to compile with a parse error. Under 8.0.2 and
 8.2.1 it parses without any trouble.

 We should sort out what the desired parse behavior for `MultiWayIf` is and
 add a test to ensure it doesn't change again.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14130
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