STDEXEC::task<void> test(asio::steady_timer& timer)
  {
    co_await timer.async_wait(asio::as_tuple(exec::asio::use_sender));
  }

fails to compile with:

[build] /usr/include/boost/asio/impl/as_tuple.hpp:301:65: fatal error: too few arguments to function call, expected 2, have 1
[build]   301 |     return Associator<Handler, DefaultCandidate>::get(h.handler_);
[build]       |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~           ^

here, Associator is asio::associated_allocator specialized for ::exec::asio::detail::completion_token::completion_handler. ASIO clearly expects the get static member function to be callable with one argument. get doesn't use its second argument, so making it optional fixes the issue.

attn @RobertLeahy