If you rename a core Var, you will get a warning if you redefine it. It is as if the :rename doesn't imply :exclude.
Repro with QuickStart JAR:
Compare to Clojure:
Note that you cannot workaround this by simply adding an explicit :exclude for map above. While this works with the current ClojureScript compiler, it breaks in Clojure, making the alias symbol clj-map unresolvable.