Named arguments like in python but more limited

[m-harel]

For years the C++ community has been waiting for named function arguments. Other software languages, like python, support it and allow the user to write more explicit code.
Unfortunately the regular C++ can't allow it and probably won't in the next versions. there are cumbersome workaround.

Let's take for example a simple convert function:
convert : (from : double, to : double) = { ....}

I have seen so many bugs for calling functions like convert with the wrong arguments order. named arguments, like in python, solve the bug before it appears.
convert(from=0.4, to=10.7)

Python also allows a different order of the parameters, but I don't think it's possible or necessary.

While in traditional C++ argument names are not part of the signature of a function, in Cpp2 it can be part of it, at least before the conversion to cpp file. The compiler can check the names before translate it to Cpp function. the user won't be allowed to define two functions with the same argument types and different names:
convert : (from : double, to : double) = {...}
convert : (to : double, from : double) = {...} // compilation error - convert(double, double) is already declared

Here are some usages and the expected result:
convert(to=12, from=7); // compilation error for wrong names.
convert(from=2, 6); // ok or error? I'm not sure but I guess it should be allowed
convert(1,3); // I guess this should be allowed

In addition it will be nice to allow force named arguments in the function declaration:

convert : ( [[named argument]] from : double, to : double) = {...}
will force the user to write from explicitly, but not "to". for example:
convert(to=12, from=7); // compilation error for wrong names.
convert(from=2, 6); // ok!
convert(1, 3); // error - from should be named explicitly
convert(1, to=3); // error - from should be named explicitly

My suggestion for regular c++ style functions, named arguments won't be allowed at all.

As a programmer, and more over as a team leader, I have seen developers spending weeks (I'm not exaggerating) for misusing a function by replacing the order of the parameters. Named arguments, and forced named arguments, can eliminate these bugs completely and save a lot of time.

[JohelEGP]

See also #202 (comment), which links to #193 (comment):

Do you already have a plan in mind for the syntax of designated initializers, @hsutter ? That might be what "forces" CPP2 to use { } for lists and aggregates anyway.

I've been deprioritizing them because they're not addressing one of the pressing problems with C++ and so can be looked at later. When I do look at them, I plan to consider them together with named arguments as a general feature, and avoid doing something special for constructors that doesn't work the same way for other function calls. So I don't think named arguments will be related to a brace syntax, if I do add them in the future for all functions arguments (not just constructor arguments).

[JohelEGP]

I think #542 should also be considered when designing this feature.

[JohelEGP]

#543 (comment) should also be considered.