[GHC] #8755: Happy parser file compilation causes panic

[GHC]

#8755: Happy parser file compilation causes panic
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       Reporter:  wting           |             Owner:
           Type:  bug             |            Status:  new
       Priority:  normal          |         Milestone:
      Component:  Compiler        |           Version:  7.6.3
       Keywords:                  |  Operating System:  Unknown/Multiple
   Architecture:  x86_64 (amd64)  |   Type of failure:  Compile-time crash
     Difficulty:  Unknown         |         Test Case:
     Blocked By:                  |          Blocking:
Related Tickets:                  |
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 Sorry I can't be more descriptive, but I'm not exactly sure what's causing
 the compile time crash. I can compile it on GHC 7.4.1, but it crashes for
 GHC 7.6.3.

 {{{
 ╭─ting at noa ~/panic  ‹python-2.7.3›  ‹master›
 ╰─➤  ghc DeclareParse.hs
 [1 of 4] Compiling Base             ( Base.hs, Base.o )
 [2 of 4] Compiling Lexer            ( Lexer.hs, Lexer.o )
 [3 of 4] Compiling Declare          ( Declare.hs, Declare.o )
 [4 of 4] Compiling DeclareParse     ( DeclareParse.hs, DeclareParse.o )
 ghc: panic! (the 'impossible' happened)
 (GHC version 7.6.3 for x86_64-unknown-linux):
     Prelude.head: empty list

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

 I've uploaded all the files here to check out:
 https://gist.github.com/wting/a6787a3f50ee41dda611

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8755>
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