The important thing with default parameters, if you have separate compilation, is that they are fixed in the callee and not in the caller. Otherwise, when the default value is changed, previously compiled callers will use a different value than declared by the callee. Compiling the default value into the caller is a problem in C++, and (IIRC) in Swift. In other words, (positional) default parameters should act like syntactic sugar for equivalent method overloads. And then actual overloads provide more flexibility for the callee implementation because it doesn’t have to represent the default case as a special in-band value.
I can see that as an implementation detail, but it need not be one. There isn't a reason that a runtime couldn't use a flag value for absent args and let the callee code set the value up on their side.
I mean, the java runtime has all sorts of crazy stuff going on w/ a security manager, to almost zero benefit. Seems easy enough to make something like this work.
