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<?hh echo (call_user_func(($x) ==> $x + 1),3 ));
based on s6LqJ
Output for hhvm-3.10.0

Process exited with code 153.
Output for 5.4.34
Parse error: syntax error, unexpected 'echo' (T_ECHO) in /in/Tn7ZA on line 3
Process exited with code 255.
Output for 5.4.0 - 5.4.32
<?hh echo (call_user_func(($x) ==> $x + 1),3 ));