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fix diagnostic for passing a fallible sender to spawn #2000

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Merged
ericniebler merged 2 commits into from
Apr 7, 2026
+25 −2
Merged

fix diagnostic for passing a fallible sender to spawn #2000

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f335293
fix diagnostic for passing a fallible sender to `spawn`
ericniebler Apr 6, 2026
9656530
don't be sloppy with type checking
ericniebler Apr 6, 2026
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27 changes: 25 additions & 2 deletions include/stdexec/__detail/__spawn.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -123,13 +123,37 @@ namespace STDEXEC

struct spawn_t
{
private:
template <class _Sender, class _Token>
using _wrapped_sender_t = decltype(__declval<_Token&>().wrap(__declval<_Sender>()));

template <class _Sender, class _Env>
using __choose_senv_t = __result_of<__spawn_common::__choose_senv, _Env, env_of_t<_Sender>>;

template <class _Sender, class _Env>
using _spawn_sndr_impl_t = __result_of<write_env, _Sender, __choose_senv_t<_Sender, _Env>>;

template <class _Sender, class _Token, class _Env>
using _spawn_sndr_t = _spawn_sndr_impl_t<_wrapped_sender_t<_Sender, _Token>, _Env>;

public:
template <sender _Sender, scope_token _Token>
void operator()(_Sender&& __sndr, _Token __tkn) const
{
return (*this)(static_cast<_Sender&&>(__sndr), static_cast<_Token&&>(__tkn), env<>{});
}

template <sender _Sender, scope_token _Token, class _Env>
void operator()(_Sender&&, _Token, _Env&&) const
{
using _spawn_sndr_t = spawn_t::_spawn_sndr_t<_Sender, _Token, _Env>;
static_assert(sender_in<_spawn_sndr_t, _Env>
&& __never_sends<STDEXEC::set_error_t, _spawn_sndr_t, _Env>,
"spawn expects a sender that cannot fail");
}

template <sender _Sender, scope_token _Token, class _Env>
requires __never_sends<STDEXEC::set_error_t, _spawn_sndr_t<_Sender, _Token, _Env>, _Env>
void operator()(_Sender&& __sndr, _Token __tkn, _Env&& __env) const
{
auto __wrapped_sender = __tkn.wrap(static_cast<_Sender&&>(__sndr));
Expand All @@ -141,8 +165,7 @@ namespace STDEXEC
auto __sender_with_env = write_env(std::move(__wrapped_sender),
__spawn_common::__choose_senv(__env, __sndr_env));

using __spawn_state_t =
__spawn_state<__raw_alloc_t, std::remove_cvref_t<_Token>, decltype(__sender_with_env)>;
using __spawn_state_t = __spawn_state<__raw_alloc_t, _Token, decltype(__sender_with_env)>;

using __traits =
std::allocator_traits<__raw_alloc_t>::template rebind_traits<__spawn_state_t>;
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