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exec::repeat_until et al.: Dependent Child Sender Support #1879

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exec::repeat_until et al.: Dependent Child Sender Support #1879

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19 changes: 17 additions & 2 deletions include/exec/repeat_until.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -255,13 +255,28 @@ namespace experimental::execution
__delete_set_value_t>,
__mbind_front_q<__values_t, _Child>::template __f>;

struct _FAILED_TO_FORM_COMPLETION_SIGNATURES
{};

struct __repeat_until_impl : __sexpr_defaults
{
template <class _Sender, class... _Env>
static consteval auto __get_completion_signatures()
{
// TODO: port this to use constant evaluation
return __completions_t<__child_of<_Sender>, _Env...>{};
using __child_t = __child_of<_Sender>;
if constexpr (::STDEXEC::__minvocable_q<__completions_t, __child_t, _Env...>)
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@ericniebler ericniebler Feb 21, 2026

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imo, better would be to change __completions_t to always be well-formed and return descriptive errors. best would be to rewrite this function to use consteval instead of TMP.

{
return __completions_t<__child_t, _Env...>{};
}
else if constexpr (sizeof...(_Env) == 0)
{
return STDEXEC::__dependent_sender<_Sender>();
}
else
{
return ::STDEXEC::__throw_compile_time_error<_FAILED_TO_FORM_COMPLETION_SIGNATURES,
::STDEXEC::_WITH_PRETTY_SENDER_<_Sender>>();
}
};

static constexpr auto __connect =
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19 changes: 19 additions & 0 deletions test/exec/test_repeat_until.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -28,9 +28,11 @@
#include <catch2/catch.hpp>

#include <concepts>
#include <cstddef>
#include <limits>
#include <memory>
#include <stdexcept>
#include <type_traits>
#include <utility>

namespace ex = STDEXEC;
Expand Down Expand Up @@ -407,4 +409,21 @@ namespace {
"Missing added set_error_t(std::exception_ptr)");
}
}

TEST_CASE("repeat_until works with a dependent sender", "[adaptors][repeat_until]")
{
std::size_t invoked = 0;
auto snd = exec::repeat_until(ex::read_env(ex::get_stop_token)
| ex::then(
[&](auto token) noexcept
{
++invoked;
return std::is_same_v<ex::never_stop_token, decltype(token)>;
}));
static_assert(ex::dependent_sender<decltype(snd)>);
auto op = ex::connect(std::move(snd), expect_void_receiver{});
CHECK(!invoked);
ex::start(op);
CHECK(invoked == 1);
}
} // namespace
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