Allow 'default async fn' to parse.

src/libsyntax/parse/parser/item.rs

@@ -825,6 +825,7 @@ impl<'a> Parser<'a> {
             self.is_keyword_ahead(1, &[
                 kw::Impl,
                 kw::Const,
+                kw::Async,
                 kw::Fn,
                 kw::Unsafe,
                 kw::Extern,

src/test/ui/specialization/issue-63716-parse-async.rs

@@ -0,0 +1,14 @@
+// Ensure that `default async fn` will parse.
+// See issue #63716 for details.
+
+// check-pass
+// edition:2018
+
+#![feature(specialization)]
+
+fn main() {}
+
+#[cfg(FALSE)]
+impl Foo for Bar {
+    default async fn baz() {}
+}