There are already open issues about that, please don't open another.

I think you are confusing the concepts of 'recursion' and 'functional programming'. Not creating side effects is the point of FP, and FP happens to use recursion. But recursion itself is simply the concept of a function calling itself, which does not at all limit it to FP.

I would definetly check ... if only the reference is pushed to the stack

Objects are always passed by reference. This code wouldn't even work otherwise, since you'd run out of stack space immediately for huge inputs.

it is generally not the idea of recursion to work with reference objects

This is a baseless statement.

Thanks for the heads up.