add testcase for issue 78600

Author: csmoe

Diff for: src/test/ui/async-await/issues/issue-78600.rs

@@ -0,0 +1,12 @@
+// edition:2018
+
+struct S<'a>(&'a i32);
+
+impl<'a> S<'a> {
+    async fn new(i: &'a i32) -> Result<Self, ()> {
+        //~^ ERROR: `async fn`
+        Ok(S(&22))
+    }
+}
+
+fn main() {}