GolfScript, cracked by Dennis
{}' !$%&()*+,-./<=>?@[\]^`|~'':'*~;
This is a code-golf challenge, after all, so why not try GolfScript?
A valid solution should be a snippet that reads two integers off the stack, adds them together and returns the result on the stack. The catch is that it should still work even after the code above has redefined almost all of the built-in GolfScript operators to do abolutely nothing. At least I left ; untouched, so you can still pop values off the stack. ;-) Your code should work on the standard GolfScript interpreter, as implemented e.g. on TIO (see link above).
Dennis' solution, like my own, relies on the rarely used feature of GolfScript that allows interpolated Ruby code in double quoted strings. We use this feature to define a new addition operator that works exactly like the built-in + operator, and then call it.
(One reason why the Ruby interpolation feature in GolfScript is so rarely used is that, awkwardly, the interpolated Ruby code is executed during parsing, and its output is cached by the GolfScript interpreter. Thus, if you e.g. have a string with interpolated Ruby code in a loop, the code will run only once before the actual program starts and thereafter always return the same value on every iteration of the loop. You can work around that using string eval to defer parsing, but that makes the already awkward syntax even more ugly and verbose, and in any case, for this challenge I disabled the eval operator ~, too. However, it turns out that defining new built-in GolfScript operatorsdefining new built-in GolfScript operators is one thing this feature actually does quite nicely and cleanly.)