Comment by doug-moen

I think that what you are calling "immutable pass by reference" is what the OP is calling "pass by value". See, when used abstractly, "pass by value" means that the argument is passed as a value, hence it is immutable and the callee can't mutate it. One way to implement this is by copying the data that represents the value. In the OP's language, and in many other languages that work this way, instead of copying the data, we implement "pass by value" by incrementing the reference count and passing a pointer to the original data. These differing implementations provide the same abstract semantics, but differ in performance.