Summary

The compiler tries to emit jvm native instanceof expressions for direct clojure.core/instance? calls.
For that, it tries to resolve its first argument as a class name. However, it disregards lexical bindings when doing that.
This is incongruent to the default implementation in core.clj

Patches

[Stu] All three patches should be applied IMO.

• 0002 makes instance? respect lexical bindings
• 0003 makes instance?'s compiled form check arity, consistent with higher-order behavior
• 0001 has a minimal test for both 0002 and 0003.

Data

Test case

user=> (let [Long String] (instance? Long "abc"))
false
;; expected true as in
user=> (let [Long String] (apply instance? [Long "abc"]))
true

Culprit method

https://github.com/clojure/clojure/blob/4ccb10edbe66eae81336a4c0972050d9759b0bf7/src/jvm/clojure/lang/Compiler.java#L3578

List Discussion

https://groups.google.com/d/topic/clojure/mf25OlFRpa8/discussion

Tangent

This was discovered because the same compiler macro also omits the arity check implicit in the default definition. This could also conveniently be fixed when touching that method:

user=> (instance? String)
false
;; expected
user=> (apply instance? [String])
ArityException Wrong number of args (1) passed to: core$instance-QMARK-  clojure.lang.AFn.throwArity (AFn.java:437)

EDIT elaborated on ticket title and description; added tangent