CLJS-2865: Optimize string expression concatenation

Borrow a bit of the logic used for boolean expressions to check if
arguments to the str macro are of string or clj-nil type, and to also
cause the str macro to indicate that its return type is string.

Author: mfikes

src/main/clojure/cljs/core.cljc

@@ -827,22 +827,35 @@
       (core/quot c 32)
       (core/inc (core/quot c 32)))))
 
+(core/defn- compatible? [inferred-tag allowed-tags]
+  (if (set? inferred-tag)
+    (clojure.set/subset? inferred-tag allowed-tags)
+    (contains? allowed-tags inferred-tag)))
+
+(core/defn- typed-expr? [env form allowed-tags]
+  (compatible? (cljs.analyzer/infer-tag env
+                 (cljs.analyzer/no-warn (cljs.analyzer/analyze env form)))
+    allowed-tags))
+
+(core/defn- string-expr [e]
+  (vary-meta e assoc :tag 'string))
+
 (core/defmacro str
   ([] "")
   ([x]
-   (if (core/string? x)
+   (if (typed-expr? &env x '#{string})
      x
-     (core/list 'js* "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})" x)))
+     (string-expr (core/list 'js* "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})" x))))
   ([x & ys]
    (core/let [interpolate (core/fn [x]
-                            (if (core/string? x)
+                            (if (typed-expr? &env x '#{string clj-nil})
                               "~{}"
                               "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})"))
               strs        (core/->> (core/list* x ys)
                             (map interpolate)
                             (interpose ",")
                             (apply core/str))]
-     (list* 'js* (core/str "[" strs "].join('')") x ys))))
+     (string-expr (list* 'js* (core/str "[" strs "].join('')") x ys)))))
 
 (core/defn- bool-expr [e]
   (vary-meta e assoc :tag 'boolean))

src/test/cljs/cljs/core_test.cljs

@@ -1637,6 +1637,15 @@
   (is (= "xyzzy" (str "x" "y" "z" "z" "y")))
   (is (= "a1b2c3" (str "a" 1 "b" 2 "c" 3))))
 
+(defn str-fn-2865 []
+  "hello")
+
+(deftest test-cljs-2865
+  (is (= "ab" (str "a" (let [x true] (when x "b")))))
+  (is (= "ab" (str "a" js/undefined "b")))
+  (is (= "ab" (str "a" nil "b")))
+  (is (= "ahellob" (str "a" (str-fn-2865) "b"))))
+
 (deftest test-cljs-2934
   (let [x (delay 1)]
     (is (= "#object[cljs.core.Delay {:status :pending, :val nil}]" (pr-str x)))