CLJS-3140: Not inferring on implements?

Author: mfikes

src/main/clojure/cljs/analyzer.cljc

@@ -1599,14 +1599,15 @@
 
 (defn- type-check-induced-tag
   "Look for a type-check-induced tag when the test expression is the use of
-   instance? on a local, as in (instance? ICounted x)."
+  instance? on a local, as in (instance? UUID x) or implements? on a local, as
+  in (implements? ICounted x)."
   [env test]
   (when (and (list? test)
              (== 3 (count test))
              (every? symbol? test))
     (let [analyzed-fn (no-warn (analyze (assoc env :context :expr) (first test)))]
       (when (= :var (:op analyzed-fn))
-        (when ('#{cljs.core/instance?} (:name analyzed-fn))
+        (when ('#{cljs.core/instance? cljs.core/implements?} (:name analyzed-fn))
           (let [analyzed-type (no-warn (analyze (assoc env :context :expr) (second test)))
                 tag (:name analyzed-type)
                 sym (last test)]

src/test/clojure/cljs/analyzer_tests.clj

@@ -2091,3 +2091,13 @@
            (env/with-compiler-env test-cenv
              (:tag (ana/analyze test-env '(let [x ^any []] (if (qualified-ident? x) x nil))))))
         '#{cljs.core/Keyword cljs.core/Symbol clj-nil})))
+
+(deftest test-cljs-3140
+  (is (= (ana/no-warn
+           (env/with-compiler-env test-cenv
+             (:tag (ana/analyze test-env '(let [x ^any []] (if (instance? UUID x) x nil))))))
+        '#{cljs.core/UUID clj-nil}))
+  (is (= (ana/no-warn
+           (env/with-compiler-env test-cenv
+             (:tag (ana/analyze test-env '(let [x ^any []] (if (implements? ICounted x) x nil))))))
+        '#{cljs.core/ICounted clj-nil})))