add testcase for issue-70818

Author: csmoe

src/test/ui/async-await/issue-70818.rs

@@ -0,0 +1,5 @@
+// edition 2018
+
+fn d<T: Sized>(t: T) -> impl std::future::Future<Output = T> + Send { //~ Error `T` cannot be sent between threads safely
+    async { t }
+}