CLJS-3320: Compiler warning on trying to use js as an ns (#224)

Co-authored-by: Mike Fikes <mike@fikesfarm.com>

Author: swannodette

Diff for: src/main/clojure/cljs/analyzer.cljc

@@ -159,6 +159,7 @@
    :protocol-impl-recur-with-target true
    :single-segment-namespace true
    :munged-namespace true
+   :js-used-as-alias true
    :ns-var-clash true
    :non-dynamic-earmuffed-var true
    :extend-type-invalid-method-shape true
@@ -439,6 +440,10 @@
     (str "Namespace " name " contains a reserved JavaScript keyword,"
          " the corresponding Google Closure namespace will be munged to " munged)))
 
+(defmethod error-message :js-used-as-alias
+  [warning-type {:keys [spec] :as info}]
+  (str "In " (pr-str spec) ", the alias name js is reserved for JavaScript interop"))
+
 (defmethod error-message :ns-var-clash
   [warning-type {:keys [ns var] :as info}]
   (str "Namespace " ns " clashes with var " var))
@@ -2994,6 +2999,9 @@
                 lib'       ((alias-type @aliases) alias)]
             (when (and (some? lib') (not= lib lib'))
               (throw (error env (parse-ns-error-msg spec ":as alias must be unique"))))
+            (when (= alias 'js)
+              (when-not (= lib (get-in @aliases [(if macros? :fns :macros) 'js])) ; warn only once
+                (warning :js-used-as-alias env {:spec spec})))
             (swap! aliases
               update-in [alias-type]
               conj [alias lib] (when js-module-provides [js-module-provides lib]))))

Diff for: src/test/clojure/cljs/analyzer_tests.clj

@@ -1493,6 +1493,13 @@
             (:import goog))]))
     (is (= {} (get-in @cenv [::ana/namespaces 'test.foo :imports])))))
 
+(deftest test-cljs-3320
+  (let [ws (atom [])]
+    (ana/with-warning-handlers [(collecting-warning-handler ws)]
+      (binding [ana/*cljs-ns* 'cljs.user]
+        (analyze ns-env '(ns cljs3320.core (:require [cljs.js :as js])))))
+    (is (string/includes? (first @ws) "the alias name js is reserved for JavaScript interop"))))
+
 (deftest test-cljs-3371
   (let [ws (atom [])]
     (ana/with-warning-handlers [(collecting-warning-handler ws)]