fix corner case in reduce_vars (#4384)

fixes #4383

lib/compress.js

@@ -644,7 +644,14 @@ merge(Compressor.prototype, {
                     fixed = save;
                     return true;
                 }
-                visit(node, fixed);
+                visit(node, fixed, function() {
+                    var save_len = tw.stack.length;
+                    for (var i = 0, len = scanner.stack.length - 1; i < len; i++) {
+                        tw.stack.push(scanner.stack[i]);
+                    }
+                    node.walk(tw);
+                    tw.stack.length = save_len;
+                });
                 return true;
             });
             lhs.walk(scanner);
@@ -678,10 +685,10 @@ merge(Compressor.prototype, {
                 node.right.walk(tw);
                 scan_declaration(tw, left, function() {
                     return node.right;
-                }, function(sym, fixed) {
+                }, function(sym, fixed, walk) {
                     if (!(sym instanceof AST_SymbolRef)) {
                         mark_assignment_to_arguments(sym);
-                        sym.walk(tw);
+                        walk();
                         return;
                     }
                     var d = sym.definition();
@@ -696,7 +703,7 @@ merge(Compressor.prototype, {
                         sym.fixed = d.fixed = fixed;
                         sym.fixed.assigns = [ node ];
                     } else {
-                        sym.walk(tw);
+                        walk();
                         d.fixed = false;
                     }
                 });

test/compress/destructured.js

@@ -1844,3 +1844,25 @@ issue_4372_2: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4383: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unsafe: true,
+    }
+    input: {
+        console.log(function(a) {
+            [ a[0] ] = [];
+            return a.length;
+        }([]));
+    }
+    expect: {
+        console.log(function(a) {
+            [ a[0] ] = [];
+            return a.length;
+        }([]));
+    }
+    expect_stdout: "1"
+    node_version: ">=6"
+}