void FooFunction(int x, int y, optional<int> optarg1 = {}, optional<int> optarg2 = {}) {
     int arg1 = optarg1.value_or(3);
     int arg2 = optarg1.value_or(7);
   }

   void usage_code() {
     FooFunction(x, y, {}, 9);
   }

I.e. for complex interfaces defaulted arguments should default to an out-of-band placeholder, not to the actual value.

I do like the struct as well, but it is still not ideal if you want to use initializers. I.e this doesn't work in C++:

   FooFunction(x, y, {.arg2 = 9});

You have to specify all preceding values FooFunction(x, y, {.arg1 = 2, .arg2 = 9});

Works better with optional (and converting everything to a struct):

   FooFunction({.x = x, .y=x, .arg1 = nullopt, .arg2 = 9});