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Say so when a sender adaptor closure is used as a sender #2378

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Merged
ericniebler merged 1 commit into from
Oct 30, 2026
Merged

Say so when a sender adaptor closure is used as a sender #2378

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17 changes: 16 additions & 1 deletion include/stdexec/__detail/__diagnostic_macros.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,7 @@
"\n" \
"\n" \
"The given type is not a sender because " STDEXEC_PP_STRINGIZE(STDEXEC) "::enable_sender<Sender>"\
"is false. Either:\n" \
" is false. Either:\n" \
"\n" \
"1. Give the type a nested '::sender_concept' type that is an alias for '" \
STDEXEC_PP_STRINGIZE(STDEXEC) "::sender_tag',\n" \
Expand All @@ -50,6 +50,21 @@
" template <>\n" \
" inline constexpr bool " STDEXEC_PP_STRINGIZE(STDEXEC) "::enable_sender<MySender> = true;\n"

////////////////////////////////////////////////////////////////////////////////
#define STDEXEC_ERROR_SENDER_ADAPTOR_CLOSURE_IS_NOT_A_SENDER \
"\n" \
"\n" \
"The given type is a sender adaptor closure, not a sender. A closure such as\n" \
"'" STDEXEC_PP_STRINGIZE(STDEXEC) "::then(fn)' needs a sender to adapt.\n" \
"\n" \
"Either pipe a sender into it:\n" \
"\n" \
" auto sndr2 = sndr | " STDEXEC_PP_STRINGIZE(STDEXEC) "::then(fn);\n" \
"\n" \
"or pass the sender as the first argument:\n" \
"\n" \
" auto sndr2 = " STDEXEC_PP_STRINGIZE(STDEXEC) "::then(sndr, fn);\n"

////////////////////////////////////////////////////////////////////////////////
#define STDEXEC_ERROR_CANNOT_COMPUTE_COMPLETION_SIGNATURES \
"\n" \
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12 changes: 11 additions & 1 deletion include/stdexec/__detail/__sender_concepts.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -294,10 +294,20 @@ namespace STDEXEC

namespace __detail
{
// sender_adaptor_closure only accepts class types. In a concept, a type it
// rejects (e.g. int) makes the check false instead of a hard error.
template <class _Sender>
concept __is_sender_adaptor_closure =
__std::derived_from<__decay_t<_Sender>, sender_adaptor_closure<__decay_t<_Sender>>>;

template <class _SenderName, class _Sender, class... _Env>
constexpr auto __diagnose_sender_concept_failure() noexcept
{
if constexpr (!enable_sender<__decay_t<_Sender>>)
if constexpr (!enable_sender<__decay_t<_Sender>> && __is_sender_adaptor_closure<_Sender>)
{
static_assert(enable_sender<_Sender>, STDEXEC_ERROR_SENDER_ADAPTOR_CLOSURE_IS_NOT_A_SENDER);
}
else if constexpr (!enable_sender<__decay_t<_Sender>>)
{
static_assert(enable_sender<_Sender>, STDEXEC_ERROR_ENABLE_SENDER_IS_FALSE);
}
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