...one of the most highly
regarded and expertly designed C++ library projects in the
world. — Herb Sutter and Andrei
constexpr as_single_t( default_constructor_tag = default_constructor_tag(), CompletionToken token = CompletionToken());
This constructor is only valid if the underlying completion token is default constructible and move constructible. The underlying completion token is itself defaulted as an argument to allow it to capture a source location.