Happy parser file compilation causes panic

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@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

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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