Protocols do not work like Java interfaces or classes. Their methods are compiled into regular functions which lookup the implementation to use at runtime based on the runtime type of the receiver. Compilation will check for the named function but doesn't do any further checking. Given the following protocol and implementation:
(defprotocol P
(method [this ^Integer i]))
(extend-protocol P
String
(method [s i] (.substring s i)))
both (method "test" "call") and (method 1 2) will be accepted by the compilation phase but will fail at runtime.
Of course there's no requirement for Clojure code to be AOT compiled anyway so in that case any name errors will still only be caught at runtime when the compilation happens.
Type hinted bindings are only converted into a cast and are not checked at compilation time either e.g.
(defn hinted [^String s] (.length s))
(hinted 3)
will be accepted but fail at runtime.
deftype is only used for Java interop an is also not a form of type checking. The methods will be compiled into Java classes and interfaces, but the implementations defer to regular Clojure functions which are not type checked. You can only make use of the type information by referencing the compiled class files in Java or another statically typed language, using them from Clojure will not perform type checking.
