You don't. The way it works is that it intercepts the call site when the input args are constant. If they're not then it won't be replaced and it will call the original method. C# source generators can't replace method definitions, however a call site can be changed to another method via source generators.
You don't have to write the method twice. Source generators can only add new code, they cannot take away a method you declared, so it will still be there at runtime when called with non-constant arguments.
