[SUGGESTION] Allow concept type constraints with the `_` wildcard (C++ constrained `auto`)

[bluetarpmedia]

(I can't find an issue/discussion about this so apologies if I've missed it.)

Suggestion Allow concept placeholder type constraints in Cpp2 with the _ wildcard, as an equivalent of constrained auto in C++.

I'd like to write the Cpp2 equivalent of the following C++:

void print(std::ranges::forward_range auto const& r) {
  // etc
}

std::vector<int> vi = get_data();
print(vi);

This works in Cpp2 (see example):

print: <Rng>(r: Rng) requires (std::ranges::forward_range<Rng>) = {

}

But it reads quite verbose to me compared to the original C++. Something like the following would be closer to the original, I think:

print: (r: std::ranges::forward_range _) = {

}

Will your feature suggestion eliminate X% of security vulnerabilities of a given kind in current C++ code? No

Will your feature suggestion automate or eliminate X% of current C++ guidance literature? No

[hsutter]

Edited: ~Thanks, good catch! I see there isn't a similar forward algorithm so I just need to adjust this for move...~

Sorry, replied on wrong issue...

[hsutter]

Thanks! I've added is constraints on wildcarded parameters and locals.

With the above commit, the following now works (and the last line would be an error if not commented out):

print: (r: _ is std::regular) = {
    std::cout << "satisfies std::regular\n";
}

print: (_) = {
    std::cout << "fallback\n";
}

irregular: type = {}

main: () = {
    print(42);           // prints: satisfies std::regular
    print(irregular());  // prints: fallback

    ok : _ is std::regular = 42;
    //err: _ is std::regular = irregular();
}

[bluetarpmedia]

Nice work Herb!