Add the test minimized from deadpool

Author: saethlin

tests/pass/async-fn.rs

@@ -58,6 +58,20 @@ async fn hello_world() {
     read_exact(&mut reader, &mut marker).await.unwrap();
 }
 
+async fn uninhabited_variant() {
+    async fn unreachable(_: Never) {}
+
+    let c = async {};
+    match None::<Never> {
+        None => {
+            c.await;
+        }
+        Some(r) => {
+            unreachable(r).await;
+        }
+    }
+}
+
 fn run_fut<T>(fut: impl Future<Output = T>) -> T {
     use std::task::{Context, Poll, Waker};
 
@@ -80,4 +94,5 @@ fn main() {
     assert_eq!(run_fut(includes_never(false, 4)), 16);
     assert_eq!(run_fut(partial_init(4)), 8);
     run_fut(hello_world());
+    run_fut(uninhabited_variant());
 }