Better error message in ed 2015

tests/ui/async-await/async-fn/edition-2015-not-async-bound.rs

@@ -0,0 +1,10 @@
+// check-pass
+// Make sure that we don't eagerly recover `async ::Bound` in edition 2015.
+
+mod async {
+    pub trait Foo {}
+}
+
+fn test(x: impl async ::Foo) {}
+
+fn main() {}