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replace std::variant with __variant in as_awaitable #1903

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replace std::variant with __variant in as_awaitable #1903

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89b90bc
replace `std::variant` with `__variant` in `as_awaitable`
ericniebler Feb 27, 2026
d6d7b24
missed an edit
ericniebler Feb 27, 2026
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37 changes: 18 additions & 19 deletions include/stdexec/__detail/__as_awaitable.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,11 +25,11 @@
#include "__queries.hpp"
#include "__tag_invoke.hpp"
#include "__type_traits.hpp"
#include "__variant.hpp"

#include <exception>
#include <functional> // for std::identity
#include <system_error>
#include <variant>

namespace STDEXEC
{
Expand Down Expand Up @@ -81,8 +81,7 @@ namespace STDEXEC
using __value_or_void_t = __if_c<__same_as<_Value, void>, __void, _Value>;

template <class _Value>
using __expected_t =
std::variant<std::monostate, __value_or_void_t<_Value>, std::exception_ptr>;
using __expected_t = __variant<__value_or_void_t<_Value>, std::exception_ptr>;

template <class _Tag, class _Sender, class... _Env>
concept __completes_inline_for = __never_sends<_Tag, _Sender, _Env...>
Expand All @@ -103,23 +102,23 @@ namespace STDEXEC
{
STDEXEC_TRY
{
__result_.template emplace<1>(static_cast<_Us&&>(__us)...);
__result_.template emplace<0>(static_cast<_Us&&>(__us)...);
}
STDEXEC_CATCH_ALL
{
__result_.template emplace<2>(std::current_exception());
__result_.template emplace<1>(std::current_exception());
}
}

template <class _Error>
void set_error(_Error&& __err) noexcept
{
if constexpr (__decays_to<_Error, std::exception_ptr>)
__result_.template emplace<2>(static_cast<_Error&&>(__err));
__result_.template emplace<1>(static_cast<_Error&&>(__err));
else if constexpr (__decays_to<_Error, std::error_code>)
__result_.template emplace<2>(std::make_exception_ptr(std::system_error(__err)));
__result_.template emplace<1>(std::make_exception_ptr(std::system_error(__err)));
else
__result_.template emplace<2>(std::make_exception_ptr(static_cast<_Error&&>(__err)));
__result_.template emplace<1>(std::make_exception_ptr(static_cast<_Error&&>(__err)));
}

__expected_t<_Value>& __result_;
Expand All @@ -136,8 +135,8 @@ namespace STDEXEC

void set_stopped() noexcept
{
// no-op: the __result_ variant will remain engaged with the monostate
// alternative, which signals that the operation was stopped.
// no-op: the __result_ variant will remain valueless, which signals that the
// operation was stopped.
}

// Forward get_env query to the coroutine promise
Expand Down Expand Up @@ -185,7 +184,7 @@ namespace STDEXEC
}
STDEXEC_CATCH_ALL
{
this->__result_.template emplace<2>(std::current_exception());
this->__result_.template emplace<1>(std::current_exception());
this->__continuation_.resume();
}
}
Expand All @@ -207,21 +206,21 @@ namespace STDEXEC

constexpr auto await_resume() -> _Value
{
// If the operation completed with set_stopped (as denoted by the monostate
// alternative being active), we should not be resuming this coroutine at all.
STDEXEC_ASSERT(__result_.index() != 0);
if (__result_.index() == 2)
// If the operation completed with set_stopped (as denoted by the result variant
// being valueless), we should not be resuming this coroutine at all.
STDEXEC_ASSERT(!__result_.__is_valueless());
if (__result_.index() == 1)
{
// The operation completed with set_error, so we need to rethrow the exception.
std::rethrow_exception(std::move(std::get<2>(__result_)));
std::rethrow_exception(std::move(__var::__get<1>(__result_)));
}
// The operation completed with set_value, so we can just return the value, which
// may be void.
return static_cast<std::add_rvalue_reference_t<_Value>>(std::get<1>(__result_));
return static_cast<std::add_rvalue_reference_t<_Value>>(__var::__get<0>(__result_));
}

protected:
__expected_t<_Value> __result_{};
__expected_t<_Value> __result_{__no_init};
};

//////////////////////////////////////////////////////////////////////////////////////
Expand Down Expand Up @@ -269,7 +268,7 @@ namespace STDEXEC
STDEXEC::start(__opstate);
}

if (this->__result_.index() == 0)
if (this->__result_.__is_valueless())
{
// The operation completed with set_stopped, so we need to call
// unhandled_stopped() on the promise to propagate the stop signal. That will
Expand Down
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