fix corner case in ie8

fixes #4958

lib/scope.js

@@ -409,13 +409,14 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
         } else {
             new_def = scope.def_variable(node);
         }
+        if (new_def.undeclared) self.variables.set(name, new_def);
+        if (name == "arguments" && is_arguments(old_def) && node instanceof AST_SymbolLambda) return true;
         old_def.defun = new_def.scope;
         old_def.forEach(function(node) {
             node.redef = old_def;
             node.thedef = new_def;
             node.reference(options);
         });
-        if (new_def.undeclared) self.variables.set(name, new_def);
         return true;
     }
 });

test/compress/ie8.js

@@ -2997,3 +2997,23 @@ issue_4928_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4958: {
+    options = {
+        collapse_vars: true,
+        ie8: true,
+    }
+    input: {
+        console.log(function arguments(a) {
+            a = 21;
+            return arguments[0] + 21;
+        }("FAIL"));
+    }
+    expect: {
+        console.log(function arguments(a) {
+            a = 21;
+            return arguments[0] + 21;
+        }("FAIL"));
+    }
+    expect_stdout: "42"
+}