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/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

@@ -140,15 +140,24 @@ namespace {
     exec::static_thread_pool pool{2};
 
     struct throwing_copy {
-      throwing_copy() = default;
+      bool& should_throw_;
 
-      throwing_copy(const throwing_copy&) {
-        throw std::logic_error("cannot copy");
+      explicit throwing_copy(bool& should_throw) noexcept
+        : should_throw_(should_throw) {
+      }
+
+      throwing_copy(const throwing_copy& other)
+        : should_throw_(other.should_throw_) {
+        if (should_throw_) {
+          throw std::logic_error("cannot copy");
+        }
       }
     };
 
-    ex::sender auto snd =
-      scope.spawn_future(ex::starts_on(pool.get_scheduler(), ex::just(throwing_copy())));
+    bool should_throw = false;
+    ex::sender auto snd = scope.spawn_future(
+      ex::starts_on(pool.get_scheduler(), ex::just(throwing_copy(should_throw))));
+    should_throw = true;
     try {
       sync_wait(std::move(snd));
       FAIL("Exceptions should have been thrown");