fix corner case in reduce_vars (#4283)

fixes #4282

lib/compress.js

@@ -616,7 +616,11 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_DestructuredObject) {
                     var save = fixed;
                     node.properties.forEach(function(node) {
-                        if (node.key instanceof AST_Node) node.key.walk(tw);
+                        if (node.key instanceof AST_Node) {
+                            push(tw);
+                            node.key.walk(tw);
+                            pop(tw);
+                        }
                         fixed = function() {
                             var key = node.key;
                             var type = AST_Sub;

test/compress/destructured.js

@@ -541,7 +541,7 @@ funarg_reduce_vars_3: {
         (function({
             [a++]: b
         }) {})(0);
-        console.log(1);
+        console.log(a);
     }
     expect_stdout: "1"
     node_version: ">=6"
@@ -1317,3 +1317,27 @@ issue_4280: {
     expect_stdout: "undefined"
     node_version: ">=6"
 }
+
+issue_4282: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function(a) {
+            ({
+                [a = "bar"]: 0[console.log(a)],
+            } = 0);
+        })("foo");
+    }
+    expect: {
+        (function(a) {
+            ({
+                [a = "bar"]: 0[console.log(a)],
+            } = 0);
+        })("foo");
+    }
+    expect_stdout: true
+    node_version: ">=6"
+}