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Improve spawn/spawn_future diagnostics for invalid senders #2310

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28 changes: 19 additions & 9 deletions include/stdexec/__detail/__spawn.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -173,9 +173,11 @@ namespace STDEXEC
//! The argument sender must not be able to complete with @c set_error
//! — @c spawn cannot deliver an error to a non-existent caller. The
//! @c requires clause enforces this with a
//! `__never_sends<set_error_t, ...>` check; the diagnostic
//! overload says "spawn expects a sender that cannot fail" if the check
//! fires.
//! `__never_sends<set_error_t, ...>` check. The diagnostic overload first
//! checks that the sender after scope wrapping and environment injection is
//! valid in the selected environment and delegates to the sender diagnostics
//! if that check fails; otherwise, it reports that @c spawn expects a sender
//! that cannot fail.
//!
//! Successful and stopped completions are both accepted; their results
//! are discarded.
Expand Down Expand Up @@ -236,19 +238,27 @@ namespace STDEXEC
// with the primary three-argument overload above (they differ only by a
// constraint), which the Sphinx C++ domain cannot disambiguate.
# if !defined(STDEXEC_DOXYGEN_INVOKED)
//! @brief Diagnostic overload — selected when the sender's completion
//! signatures include @c set_error_t. Emits a @c static_assert
//! explaining that @c spawn expects a sender that cannot fail.
//! @brief Diagnostic overload selected when the no-error requirement fails.
//! If the sender after scope wrapping and environment injection is
//! not valid in the selected environment, delegates to the sender
//! diagnostics; otherwise, emits a @c static_assert explaining that
//! @c spawn expects a sender that cannot fail.
//!
//! Not normally called; the @c requires clause on the primary overload
//! steers compilation here on a constraint failure.
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");
if constexpr (!sender_in<_spawn_sndr_t, _Env>)
{
__diagnose_sender_concept_failure<_spawn_sndr_t, _Env>();
}
else
{
static_assert(__never_sends<STDEXEC::set_error_t, _spawn_sndr_t, _Env>,
"spawn expects a sender that cannot fail");
}
}
# endif // !defined(STDEXEC_DOXYGEN_INVOKED)

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5 changes: 5 additions & 0 deletions include/stdexec/__detail/__spawn_future.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -897,6 +897,11 @@ namespace STDEXEC
using __alloc_t = decltype(__spawn_common::__choose_alloc(__env, STDEXEC::get_env(__sndr)));
using __senv_t = decltype(__spawn_common::__choose_senv(__env, STDEXEC::get_env(__sndr)));

if constexpr (!sender_in<_Sender, __senv_t>)
{
__diagnose_sender_concept_failure<_Sender, __senv_t>();
}

using __spawn_future_state_t =
__spawn_future::__spawn_future_state<__alloc_t,
std::remove_cvref_t<_Token>,
Expand Down