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Constrain completion CPOs to non-const rvalue receivers #2311

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Merged
ericniebler merged 4 commits into from
Oct 9, 2026
Merged

Constrain completion CPOs to non-const rvalue receivers #2311

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f4509a9
Constrain completion CPOs to non-const rvalue receivers
emmett2020 Oct 7, 2026
7f0e8d5
Merge branch 'main' into fix/completion-cpo-receiver-constraints
emmett2020 Oct 7, 2026
be8962a
fix review comment
emmett2020 Oct 8, 2026
d5714ce
Merge branch 'main' into fix/completion-cpo-receiver-constraints
emmett2020 Oct 8, 2026
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6 changes: 4 additions & 2 deletions include/exec/sequence/merge_each.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1095,8 +1095,10 @@ namespace experimental::execution
using sender_concept = STDEXEC::sender_tag;

template <class _NextReceiver>
using __next_sequence_op_t =
__next_sequence_op<_NestedSequenceSender, _NextReceiver, _OperationBase, _NestedSeqOp>;
using __next_sequence_op_t = __next_sequence_op<_NestedSequenceSender,
__decay_t<_NextReceiver>,
_OperationBase,
_NestedSeqOp>;

template <class _Self, class... _Env>
static consteval auto get_completion_signatures() noexcept
Expand Down
2 changes: 1 addition & 1 deletion include/stdexec/__detail/__parallel_scheduler.hpp
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Expand Up @@ -539,7 +539,7 @@ namespace STDEXEC
auto st = get_stop_token(STDEXEC::get_env(__state_.__rcvr_));
if (st.stop_requested())
{
STDEXEC::set_stopped(__state_.__rcvr_);
STDEXEC::set_stopped(static_cast<__rcvr_t&&>(__state_.__rcvr_));
return;
}

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10 changes: 7 additions & 3 deletions include/stdexec/__detail/__receivers.hpp
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Expand Up @@ -41,6 +41,10 @@ namespace STDEXEC
{
namespace __detail
{
// For deduced forwarding-reference arguments, rejects lvalues and const rvalues.
template <class _Ty>
concept __non_const_rvalue = !STDEXEC_IS_SAME(_Ty, _Ty const);

template <__disposition _Disposition>
struct __completion_tag
{
Expand Down Expand Up @@ -114,7 +118,7 @@ namespace STDEXEC
//! `.set_value(_As...)` member.
//! @tparam _As The value-datum argument types.
template <class _Receiver, class... _As>
requires __set_value_member<_Receiver, _As...>
requires __detail::__non_const_rvalue<_Receiver> && __set_value_member<_Receiver, _As...>
STDEXEC_ATTRIBUTE(host, device, always_inline)
constexpr void operator()(_Receiver &&__rcvr, _As &&...__as) const noexcept
{
Expand Down Expand Up @@ -182,7 +186,7 @@ namespace STDEXEC
//! @tparam _Receiver A type with a matching `.set_error(_Error)` member.
//! @tparam _Error The error datum type.
template <class _Receiver, class _Error>
requires __set_error_member<_Receiver, _Error>
requires __detail::__non_const_rvalue<_Receiver> && __set_error_member<_Receiver, _Error>
STDEXEC_ATTRIBUTE(host, device, always_inline)
constexpr void operator()(_Receiver &&__rcvr, _Error &&__err) const noexcept
{
Expand Down Expand Up @@ -248,7 +252,7 @@ namespace STDEXEC
//! @tparam _Receiver A type with a matching nullary
//! `.set_stopped()` member.
template <class _Receiver>
requires __set_stopped_member<_Receiver>
requires __detail::__non_const_rvalue<_Receiver> && __set_stopped_member<_Receiver>
STDEXEC_ATTRIBUTE(host, device, always_inline)
constexpr void operator()(_Receiver &&__rcvr) const noexcept
{
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68 changes: 34 additions & 34 deletions test/stdexec/cpos/test_cpo_receiver.cpp
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Expand Up @@ -35,7 +35,7 @@ namespace

struct recv_value
{
int* target_;
int *target_;

void set_value(int val) noexcept
{
Expand All @@ -55,7 +55,7 @@ namespace

struct recv_rvalref
{
int* target_;
int *target_;

void set_value(int val) noexcept
{
Expand All @@ -75,7 +75,7 @@ namespace

struct recv_ref
{
int* target_;
int *target_;

void set_value(int val) noexcept
{
Expand All @@ -95,7 +95,7 @@ namespace

struct recv_cref
{
int* target_;
int *target_;

void set_value(int val) noexcept
{
Expand All @@ -113,6 +113,14 @@ namespace
}
};

// Const members ensure that rejection comes from the CPO constraints.
struct recv_const
{
void set_value(int) const noexcept {}
void set_error(int) const noexcept {}
void set_stopped() const noexcept {}
};

TEST_CASE("can call set_value on a void receiver", "[cpo][cpo_receiver]")
{
ex::set_value(expect_void_receiver{});
Expand Down Expand Up @@ -170,16 +178,6 @@ namespace
REQUIRE(val == 10);
}

TEST_CASE("can call set_value on a receiver with ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_value_t, recv_ref&, int>,
"cannot call set_value on recv_ref");
int val = 0;
recv_ref recv{&val};
ex::set_value(recv, 10);
REQUIRE(val == 10);
}

TEST_CASE("can call set_value on a receiver with const ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_value_t, recv_cref, int>,
Expand Down Expand Up @@ -207,16 +205,6 @@ namespace
REQUIRE(val == -10);
}

TEST_CASE("can call set_error on a receiver with ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_error_t, recv_ref&, int>,
"cannot call set_error on recv_ref");
int val = 0;
recv_ref recv{&val};
ex::set_error(recv, 10);
REQUIRE(val == -10);
}

TEST_CASE("can call set_error on a receiver with const ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_error_t, recv_cref, int>,
Expand Down Expand Up @@ -244,16 +232,6 @@ namespace
REQUIRE(val == INT_MAX);
}

TEST_CASE("can call set_stopped on a receiver with ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_stopped_t, recv_ref&>,
"cannot call set_stopped on recv_ref");
int val = 0;
recv_ref recv{&val};
ex::set_stopped(recv);
REQUIRE(val == INT_MAX);
}

TEST_CASE("can call set_stopped on a receiver with const ref type", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_stopped_t, recv_cref>,
Expand All @@ -263,6 +241,28 @@ namespace
REQUIRE(val == INT_MAX);
}

TEST_CASE("completion CPOs reject lvalue receivers", "[cpo][cpo_receiver]")
{
static_assert(!std::invocable<ex::set_value_t, recv_const &, int>);
static_assert(!std::invocable<ex::set_error_t, recv_const &, int>);
static_assert(!std::invocable<ex::set_stopped_t, recv_const &>);

static_assert(!std::invocable<ex::set_value_t, recv_const const &, int>);
static_assert(!std::invocable<ex::set_error_t, recv_const const &, int>);
static_assert(!std::invocable<ex::set_stopped_t, recv_const const &>);
}

TEST_CASE("completion CPOs reject const rvalue receivers", "[cpo][cpo_receiver]")
{
static_assert(std::invocable<ex::set_value_t, recv_const &&, int>);
static_assert(std::invocable<ex::set_error_t, recv_const &&, int>);
static_assert(std::invocable<ex::set_stopped_t, recv_const &&>);

static_assert(!std::invocable<ex::set_value_t, recv_const const &&, int>);
static_assert(!std::invocable<ex::set_error_t, recv_const const &&, int>);
static_assert(!std::invocable<ex::set_stopped_t, recv_const const &&>);
}

TEST_CASE("tag types can be deduced from set_value, set_error and set_stopped",
"[cpo][cpo_receiver]")
{
Expand Down
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