Method __get_completion_signatures

include/exec/fork_join.hpp:251–292

    template <class Self, class... Env>
    STDEXEC_ATTRIBUTE(host, device)
    static consteval auto __get_completion_signatures()
    {
      using namespace STDEXEC;

      using _closures_t   = STDEXEC::__data_of<Self>;
      using _child_sndr_t = STDEXEC::__child_of<Self>;

      if constexpr (__minvocable_q<__completion_domain_of_t, set_value_t, _child_sndr_t, Env...>)
      {
        using _domain_t            = __completion_domain_of_t<set_value_t, _child_sndr_t, Env...>;
        using _child_t             = __copy_cvref_t<Self, _child_sndr_t>;
        using _child_completions_t = __completion_signatures_of_t<_child_t, __fwd_env_t<Env>...>;
        using __decay_copyable_results_t =
          STDEXEC::__decay_copyable_results_t<_child_completions_t>;

        if constexpr (!STDEXEC::__valid_completion_signatures<_child_completions_t>)
        {
          return _child_completions_t{};
        }
        else if constexpr (!__decay_copyable_results_t::value)
        {
          return _ERROR_<_WHAT_(PREDECESSOR_RESULTS_ARE_NOT_DECAY_COPYABLE),
                         _IN_ALGORITHM_(exec::fork_join_t)>();
        }
        else
        {
          using _sndr_t =
            fork_join_impl_t::_when_all_sndr_t<_child_completions_t, _closures_t, _domain_t>;
          return __completion_signatures_of_t<_sndr_t, __fwd_env_t<Env>...>{};
        }
      }
      else if constexpr (sizeof...(Env) == 0)
      {
        return STDEXEC::__throw_dependent_sender_error<Self>();
      }
      else
      {
        return STDEXEC::__throw_compile_time_error<_INVALID_ARGUMENTS_TO_FORK_JOIN_,
                                                   __children_of<Self, __qq<_WITH_PRETTY_SENDERS_>>,
                                                   __fn_t<_WITH_ENVIRONMENT_, Env>...>();
      }
    }

    static constexpr auto __connect =
      []<class _Receiver, class _Sender>(_Sender&& __sndr, _Receiver&& __rcvr) noexcept