Why C++ does not allow function parameters used for default values latter parameters?

Question

This is a follow-up on this question. The code in the OP question there looked quite reasonable and unambiguous to me. Why does not C++ allow using former parameters to defi

Answer 1

For one thing, this would require that a is evaluated before b, but C++ (like C) does not define the order of evaluation for function parameters.

You can still get the effect you want by adding an overload:

int foo(int a, int b)
{ /* do something */ }

int foo(int a)
{ return foo(a, a); }