handle duplicate argument names in collapse_vars (#2215)

lib/compress.js

@@ -820,7 +820,11 @@ merge(Compressor.prototype, {
                     && !fn.uses_eval
                     && (iife = compressor.parent()) instanceof AST_Call
                     && iife.expression === fn) {
-                    fn.argnames.forEach(function(sym, i) {
+                    var names = Object.create(null);
+                    for (var i = fn.argnames.length; --i >= 0;) {
+                        var sym = fn.argnames[i];
+                        if (sym.name in names) continue;
+                        names[sym.name] = true;
                         var arg = iife.args[i];
                         if (!arg) arg = make_node(AST_Undefined, sym);
                         else {
@@ -840,11 +844,11 @@ merge(Compressor.prototype, {
                             });
                             arg.walk(tw);
                         }
-                        if (arg) candidates.push(make_node(AST_VarDef, sym, {
+                        if (arg) candidates.unshift(make_node(AST_VarDef, sym, {
                             name: sym,
                             value: arg
                         }));
-                    });
+                    }
                 }
             }
 

test/compress/collapse_vars.js

@@ -2320,3 +2320,25 @@ issue_2203_2: {
     }
     expect_stdout: "PASS"
 }
+
+duplicate_argname: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        function f() { return "PASS"; }
+        console.log(function(a, a) {
+            f++;
+            return a;
+        }("FAIL", f()));
+    }
+    expect: {
+        function f() { return "PASS"; }
+        console.log(function(a, a) {
+            f++;
+            return a;
+        }("FAIL", f()));
+    }
+    expect_stdout: "PASS"
+}