On Sun, Mar 6, 2022 at 10:26 AM David Jones via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

Consider the function

void foo(int i = 1, int j = 2, int k = 3);

Trailing default argument values can be accepted implicitly by simply skipping them in the function call:

foo();       //Accept default values for i, j, and k

foo(1);     //Accept default values for j and k, supply value for i

foo(1,2);  //Accept default value for k, supply values for i and j

But to my knowledge, there is no method for implicitly accepting leading default argument values - they must be copied out explicitly:

foo(1,2,5);  //Accept default values for i and j by copying them out explicitly, supply value for k

I suggest that this could be done implicitly thus:

foo(,,2); //Accept default values for i and j, supply value for k

foo(,1);  //Accept default values for i and k, supply value for j

That would close the door on the vastly vastly more useful Python-style syntax

    x = f(

        1,

        2,

    )

which in C++ today means "syntax error," but in Python and other modern languages means the same as "x = f(1, 2)".

Under your proposal, it would have to mean the same as "x = f(1, 2, /*some defaulted thing*/)".