Support for co_yield

[Xottab-DUTY]

As far as I understand the coroutines (I'm new to them), co_yield requires a promise to have yield_value function implementation. I don't see that it's there, but the yielding functionality could be useful, so here's a feature request!

A usage case could be waiting for an asynchronous IO.

[ConorWilliams]

Interesting idea, what semantics would you like a yield to have?

[Xottab-DUTY]

Maybe a fibonacci generator from here can serve as a reference: https://en.cppreference.com/w/cpp/language/coroutines

I tried to write an example case of async IO but failed with how it should look, i.e. with the semantics :))

[ConorWilliams]

Here is the example:

Generator<std::uint64_t> fibonacci_sequence(unsigned n)
{
    if (n == 0)
        co_return;
 
    if (n > 94)
        throw [std::runtime_error](http://en.cppreference.com/w/cpp/error/runtime_error)("Too big Fibonacci sequence. Elements would overflow.");
 
    co_yield 0;
 
    if (n == 1)
        co_return;
 
    co_yield 1;
 
    if (n == 2)
        co_return;
 
    [std::uint64_t](http://en.cppreference.com/w/cpp/types/integer) a = 0;
    [std::uint64_t](http://en.cppreference.com/w/cpp/types/integer) b = 1;
 
    for (unsigned i = 2; i < n; ++i)
    {
        [std::uint64_t](http://en.cppreference.com/w/cpp/types/integer) s = a + b;
        co_yield s;
        a = b;
        b = s;
    }
}

What do you expect to happen at the yield wrt fork/join?