co/MultiResume: add assertion verifying that all waiters have been re…

…sumed

src/co/MultiResume.hxx

@@ -16,7 +16,8 @@ namespace Co {
  * never "ready", it always suspends waiters.  All waiters can be
  * resumed with one method call.
  *
- * This object must remain valid until all waiters have been resumed.
+ * This object must remain valid until all waiters have been resumed
+ * (or canceled).
  *
  * This is similar to #MultiAwaitable, but there is no internal task.
  */
@@ -77,6 +78,10 @@ public:
 	[[nodiscard]]
 	MultiResume() noexcept = default;
 
+	~MultiResume() noexcept {
+		assert(requests.empty());
+	}
+
 	/**
 	 * Creates a new awaitable
 	 */