I haven't explained what it does in the first mail so i will explain now.

This proposal introduces more ways to overload a function.

What is `void f(int x, int y) {}` supposed to do differently from `void f(int x; int y) {}` or `void f(int x: int y) {}`?
In this case for the machine there is no difference other than the name mangling.

Why is it useful?

Given the code
void foo(int a=1000696967, int b=1000696969);
In c++20 there is no way to call `foo` specifying a value for `b` but not for `a`

With this proposal you will be allowed to do it in the following way
void foo(int a=1000696967; int b=1000696969);
foo(; 7); // calling foo with default valute for `a` and `b` = 7

In c++20 you can't have multiple parameters packs in a function, the only solution I know is from the std::pair constructor but it's too long to call
std::pair<std::complex<double>, std::string> p6(  // from https://en.cppreference.com/w/cpp/utility/pair/pair
                    std::forward_as_tuple(0.123, 7.7),
                    std::forward_as_tuple(10, 'a'));

 The new syntax would be
std::pair<std::complex<double>, std::string> p6(0.123, 7.7; 10, 'a');

The colon has been included just for the slice because other possibilities didn't look as good
arr[i:j] // slicing an array
arr[i, j] // indexing a matrix
arr[i; j] // does not mean anything to me

Il giorno gio 9 lug 2020 alle ore 16:02 Arthur O'Dwyer via Std-Proposals <std-proposals@lists.isocpp.org> ha scritto:
On Thu, Jul 9, 2020 at 9:32 AM Jake Arkinstall via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
The why is clear.

FWIW, no it's not. The OP didn't give any reason for the new syntax at all. In fact, the OP didn't even say what the new syntax is supposed to do!
What is `void f(int x, int y) {}` supposed to do differently from `void f(int x; int y) {}` or `void f(int x: int y) {}`? Explaining what the change does is an important part of any change request.
It's a way of forming groupings of parameters, which currently isn't possible without bringing structs into the equation. This opens up some decent possibilities: Defaults can apply to the group (rather than having to put them at the end of the call), parameter packs can be logically separated in a way they currently cannot, and so we can pass multiple parameters to operator[] without interfering with the comma operator.

FYI, there's already a deploy plan in motion (to the admittedly limited extent that WG21 can be said to make "deploy plans") to get multi-parameter `operator[]` overloads. The first step in the plan was committed in C++20:
The second step, targeted for C++23, is to support overloaded `operator[](int x, int y)` (or whatever parameter types) with the natural meaning and call syntax.
The goal and justification, targeted also for C++23 as far as I know, is to finally ship `mdspan`. (Which I personally think is a silly end goal, but there seem to be a lot of people looking forward to it just as a lot of people look forward to C++20's `span`.)

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