[GHC] #9975: RecordWildCards and PatternSynonyms cause impossible bug

11 Jan 2015

      #9975: RecordWildCards and PatternSynonyms cause impossible bug
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              Reporter:  gamegoblin  |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.4
              Keywords:              |  Operating System:  MacOS X
  RecordWildCards PatternSynonyms    |   Type of failure:  Compile-time
          Architecture:  x86_64      |  crash
  (amd64)                            |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 When using RecordWildCards with PatternSynonyms, I have found a way to
 cause this bug:

 $ /usr/local/bin/ghc-7.10.0.20141222 test.hs
 [1 of 1] Compiling Main             ( test.hs, test.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.0.20141222 for x86_64-apple-darwin):
         find_tycon
   Test
   [Test defined at test.hs:6:9,
    Test parent:Test defined at test.hs:4:13]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

 Here is the full code that causes it:

 {{{#!hs
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE PatternSynonyms #-}
 data Test = Test { x :: Int }
 pattern Test wat = Test { x = wat }
 }}}

 If you remove RecordWildCards, the bug does not happen.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9975
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