test: pin read_env's completion-behavior claims for get_scheduler() (#2098)

test/stdexec/queries/test_get_completion_behavior.cpp

@@ -61,3 +61,41 @@ static_assert(cb::__may_be_asynchronous(cb::__asynchronous_affine));
 static_assert(cb::__may_be_asynchronous(cb::__asynchronous | cb::__inline_completion));
 static_assert(cb::__may_be_asynchronous(cb::__asynchronous_affine | cb::__asynchronous));
 static_assert(cb::__may_be_asynchronous(cb::__asynchronous_affine | cb::__inline_completion));
+
+// read_env completes in its start(), so get_scheduler() may claim inline completion
+// for set_value_t -- but only for the tags it can actually send. Advertising
+// inline completion for a tag it never sends made a task that co_awaits
+// get_scheduler() | let_value(schedule) look like it completed inline, and the
+// awaiter then kept the composed operation state in its own stack frame while a
+// nested schedule() operation was still queued (see #2098).
+using __get_scheduler_sndr_t  = decltype(ex::get_scheduler());
+using __get_scheduler_attrs_t = ex::env_of_t<__get_scheduler_sndr_t>;
+
+static_assert(ex::__completes_inline<ex::set_value_t, __get_scheduler_attrs_t>);
+static_assert(!ex::__completes_inline<ex::set_error_t, __get_scheduler_attrs_t>);
+static_assert(!ex::__completes_inline<ex::set_stopped_t, __get_scheduler_attrs_t>);
+
+// The same expression from #2098, composed: the successor sender schedule(sch) may
+// complete asynchronously, so the composition must not be treated as completing
+// inline.
+inline constexpr auto __reschedule = [](auto __sch)
+{
+  return ex::schedule(__sch);
+};
+inline constexpr auto __reschedule_on_current = ex::get_scheduler() | ex::let_value(__reschedule);
+
+// For co_await, a sender is treated as completing inline only if _all_ of its
+// completion tags complete inline (see __as_awaitable::__completes_inline). The
+// nested schedule(sch) does not complete inline for set_stopped_t, so the
+// composition must not be classified as completing inline. We check the stopped
+// tag directly; the value tag of the composition _does_ complete inline
+// (let_value forwards read_env's inline value completion), so that is pinned
+// here as well. This check uses only names exported by the stdexec module: the
+// __as_awaitable namespace is not part of the module's public surface, and
+// naming it fails the modular and nvc++ configurations.
+static_assert(ex::__completes_inline<ex::set_value_t,
+                                     ex::env_of_t<decltype(__reschedule_on_current)>,
+                                     ex::env<>>);
+static_assert(!ex::__completes_inline<ex::set_stopped_t,
+                                      ex::env_of_t<decltype(__reschedule_on_current)>,
+                                      ex::env<>>);