fix a bad test for async_scope::spawn_future

the test was relying on a bug in the implementation of `__tuple`,
which this commit also fixes. finally, it fixes a bug in the
implementation of `starts_on` that causes `terminate` to be called
if moving the sender throws.

Author: ericniebler

include/exec/just_from.hpp

@@ -59,7 +59,9 @@ namespace exec {
       template <class... Ts>
         requires stdexec::__sigs::__is_compl_sig<_set_tag_t(Ts...)>
       auto operator()(Ts&&... ts) const noexcept //
-        -> stdexec::completion_signatures<_set_tag_t(Ts...)>;
+        -> stdexec::completion_signatures<_set_tag_t(Ts...)> {
+        return {};
+      }
     };
 
     template <class Rcvr>

include/stdexec/__detail/__starts_on.hpp

@@ -37,7 +37,7 @@ namespace stdexec {
     struct __always {
       _Ty __val_;
 
-      auto operator()() noexcept -> _Ty {
+      STDEXEC_ATTRIBUTE((always_inline)) constexpr auto operator()() noexcept(__nothrow_constructible_from<_Ty, _Ty>) -> _Ty {
         return static_cast<_Ty&&>(__val_);
       }
     };

include/stdexec/__detail/__tuple.hpp

@@ -46,7 +46,8 @@ namespace stdexec {
 
     template <class _Ty>
     concept __empty = //
-      STDEXEC_IS_EMPTY(_Ty) && STDEXEC_IS_TRIVIALLY_CONSTRUCTIBLE(_Ty);
+      STDEXEC_IS_EMPTY(_Ty) && STDEXEC_IS_TRIVIALLY_CONSTRUCTIBLE(_Ty)
+      && STDEXEC_IS_TRIVIALLY_COPYABLE(_Ty);
 
     template <__empty _Ty>
     inline _Ty __value{};

test/exec/async_scope/test_spawn_future.cpp

@@ -1,6 +1,7 @@
 #include <catch2/catch.hpp>
 #include <exec/async_scope.hpp>
 #include <exec/env.hpp>
+#include <exec/just_from.hpp>
 #include <exec/static_thread_pool.hpp>
 #include "test_common/schedulers.hpp"
 #include "test_common/receivers.hpp"
@@ -148,7 +149,9 @@ namespace {
     };
 
     ex::sender auto snd =
-      scope.spawn_future(ex::starts_on(pool.get_scheduler(), ex::just(throwing_copy())));
+      scope.spawn_future(ex::starts_on(pool.get_scheduler(), exec::just_from([](auto sink) {
+                                         return sink(throwing_copy());
+                                       })));
     try {
       sync_wait(std::move(snd));
       FAIL("Exceptions should have been thrown");