The optimization is often possible even if the computer does not see the call, because most (all?) ABIs have always required hidden pointer parameters for class types with non-trivial destructors.

https://godbolt.org/z/9WvnEvEYh Note how `std::unique_ptr<int>` effectively passed as a `int**`; and that the by-value unique_ptr is not destroyed at the end of the function -- destroying parameters is instead the caller's job (and commonly only happens at the end of the full expression containing the call -- though this choice is implementation-defined). But that can only work if the caller can see the updated value of the parameter (to avoid double-free for `clear`) -> thus the need to pass the parameter by hidden pointer.