That was surprisingly not that simple...
(instance? js/RegExp obj) compiles to `cljs.core.instance_QMARK_.call(null, RegExp, obj)` which also triggers the GClosure warning. So I thought I could inline `instance?` with a macro, but the instanceof operator's operands are in reverse order of instance?'s arguments. To preserve evaluation order, the expansion would be `(let t# ~t ('js* "({} instanceof ~{})" ~o t#))) however even that triggers the warning because the output leaves RegExp as an expression by itself again. I wound up just special-casing simple instanceof checks against symbols in the macro.
Attached patch fixes printing of RegExp objects and also enables calling (instance? js/RegExp X) without triggering the warning. Sadly, any more complex expression which evaluates to js/RegExp, or any higher order usage of instance? against that type will still trigger the warning.
Defect
Resolved
Minor
The simplest solution is to add a RegExp case to pr-seq.