[yatac42]

> The variables used in the expressions might not be in scope (in fact they usually aren't).

That's solved easily enough by evaluating the expression in the scope where it was defined (again that's what other languages do).

> Also, I'm rather sure that's how it works in C++

Default arguments in C++ work as I described: If you define `void f(int arg = g(x)) {}`, then calling `f()` n times will call `g` n times as well (using the `x` that was in scope when `f` was defined) - and if you never call `f`, `g` isn't called either.

An example demonstrating this: https://ideone.com/vs26Oq