Test async fn

Author: Nadrieril

tests/ui/rfcs/rfc-0000-never_patterns/diverges.rs

@@ -1,3 +1,4 @@
+// edition: 2018
 // check-pass
 #![feature(never_patterns)]
 #![allow(incomplete_features)]
@@ -27,3 +28,8 @@ fn never_match() -> u32 {
     }
     println!(); // Ensures this typechecks because of divergence.
 }
+
+// Divergence is not detected but at least nothing weirder happens.
+async fn async_never(!: Void) -> u32 {
+    0
+}