CLJS-2247: Warn when overwriting protocol method

Author: mfikes

src/main/clojure/cljs/analyzer.cljc

@@ -148,6 +148,7 @@
    :protocol-duped-method true
    :protocol-multiple-impls true
    :protocol-with-variadic-method true
+   :protocol-with-overwriting-method true
    :protocol-impl-with-variadic-method true
    :protocol-impl-recur-with-target true
    :single-segment-namespace true
@@ -398,6 +399,14 @@
   (str "Protocol " (:protocol info) " declares method "
        (:name info) " with variadic signature (&)"))
 
+(defmethod error-message :protocol-with-overwriting-method
+  [warning-type info]
+  (let [overwritten-protocol (-> info :existing :protocol)]
+    (str "Protocol " (:protocol info) " is overwriting "
+      (if overwritten-protocol "method" "function")
+      " " (:name info)
+      (when overwritten-protocol (str " of protocol " (name overwritten-protocol))))))
+
 (defmethod error-message :protocol-impl-with-variadic-method
   [warning-type info]
   (str "Protocol " (:protocol info) " implements method "

src/main/clojure/cljs/core.cljc

@@ -2121,6 +2121,11 @@
                                        (cljs.analyzer/warning
                                         :protocol-with-variadic-method
                                         &env {:protocol psym :name fname}))
+                                 _ (core/when-some [existing (core/get (-> &env :ns :defs) fname)]
+                                     (core/when-not (= p (:protocol existing))
+                                       (cljs.analyzer/warning
+                                         :protocol-with-overwriting-method
+                                         {} {:protocol psym :name fname :existing existing})))
                                  slot (symbol (core/str prefix (munge (name fname))))
                                  fname (vary-meta fname assoc
                                          :protocol p

src/test/clojure/cljs/analyzer_tests.clj

@@ -1875,6 +1875,18 @@
                :with-core? true})]
     (is (zero? (count @ws)))))
 
+(deftest test-cljs-2247
+  (let [ws (atom [])]
+    (try
+      (a/with-warning-handlers [(collecting-warning-handler ws)]
+        (e/with-compiler-env (assoc @test-cenv :repl-env {})
+          (a/analyze (ana/empty-env)
+            '(defn -foo []))
+          (a/analyze (ana/empty-env)
+            '(defprotocol IAlpha (-foo [this])))))
+      (catch Exception _))
+    (is (= ["Protocol IAlpha is overwriting function -foo"] @ws))))
+
 (deftest test-cljs-2385-infer-priority
   (let [ws  (atom [])
         res (infer-test-helper