cljs.spec.alpha/fn-sym was introduced with https://github.com/clojure/clojurescript/commit/20ba8ef9415b46c18172a59cfe63ad16d2a35a3c but wasn't meant to be public API. It should be converted to a defn-.
LGTM and passes CI.
fixed https://github.com/clojure/clojurescript/commit/e005b987a8d1fe96e76c2975b931575fc2bdfce3