fix corner case in merge_vars (#4302)

fixes #4301

lib/compress.js

@@ -4635,14 +4635,27 @@ merge(Compressor.prototype, {
                 var lhs = node.left;
                 if (lhs instanceof AST_Destructured) {
                     node.right.walk(tw);
-                    lhs.mark_symbol(function(node) {
+                    var marker = new TreeWalker(function(node) {
-                        if (node instanceof AST_SymbolRef) {
+                        if (node instanceof AST_Destructured) return;
+                        if (node instanceof AST_DestructuredKeyVal) {
+                            if (node.key instanceof AST_Node) {
+                                push();
+                                node.key.walk(tw);
+                                pop();
+                                push();
+                                node.value.walk(marker);
+                                pop();
+                            } else {
+                                node.value.walk(marker);
+                            }
+                        } else if (node instanceof AST_SymbolRef) {
                             mark(node, false, true);
                         } else {
                             node.walk(tw);
                         }
                         return true;
-                    }, tw);
+                    });
+                    lhs.walk(marker);
                     return true;
                 }
                 if (lhs instanceof AST_SymbolRef) {

test/compress/destructured.js

@@ -1559,3 +1559,35 @@ issue_4298: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4301: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        try {
+            console.log(function() {
+                var a, b = console;
+                return {
+                    [a = b]: a.p,
+                } = "foo";
+            }());
+        } catch (e) {
+            console.log("bar");
+        }
+    }
+    expect: {
+        try {
+            console.log(function() {
+                var a, b = console;
+                return {
+                    [a = b]: a.p,
+                } = "foo";
+            }());
+        } catch (e) {
+            console.log("bar");
+        }
+    }
+    expect_stdout: true
+    node_version: ">=6"
+}