The interface of race (select)

[akrzemi1]

The interface of select is not clearly documented, so it is difficult to figure out all the available signatures and return types. But judging from the rose in the documentation it is non-uniform.

The non-uniform interface of select() (spacial-case for all-void) is problematic and may cause unnecessary difficulties and surprises in generic contexts.

template <typename T, typename U> // is T a void?
promise<void> f(promise<T> pt, promise<U> pu)
{
  auto r = co_await select(pt, pu);
  r;  // which interface should I use?
      // variant or naked size_t?
}

Same goes for vector:

template <typename T> // is T a void?
promise<void> f(vector<<promise<T>> pv)
{
  auto r = co_await select(pv);
  r;  // which interface should I use?
      // pair or naked size_t?
}

Different interface calls for a different name. If promise<void> is orders of magnitude more frequent than promise<anything_else> then offer name selcetv for funcitons returning variant/pair. also, having two different functions would make it easier to document them.

Second issue. Because the documentation is too scarce, I cannot figure out what is returned by the select that is given a zero-sized vector of promises.

Third. Why do you use boost::variant2 rather than std::variant? I would expect a rationale in the docs.