That reminds me of the expression in C that will evaluate to 0 or 1 depending on whether or not "//" style comments are supported:

  int x = 1 //*
          //*/2
          ;
 

Without "//" comments this is whitespace equivalent to

  int x = 1 / 2 ;

With "//" comments this is whitespace equivalent to

  int x = 1 ;

[edit]

Full working demo: https://gist.github.com/jasom/41ab3712d83d7edbdc03bb8e43a7af...