fix corner case in merge_vars (#4299)

fixes #4298

lib/compress.js

@@ -4749,9 +4749,13 @@ merge(Compressor.prototype, {
                         var def = ref.definition();
                         var ldef = node.variables.get(ref.name);
                         if (ldef && (ldef === def || def.undeclared || node.parent_scope.find_variable(ref) === def)) {
+                            references[def.id] = false;
                             references[ldef.id] = false;
                         } else {
+                            var save = segment;
+                            pop();
                             mark(ref, true, false);
+                            segment = save;
                         }
                         return true;
                     });

test/compress/destructured.js

@@ -1490,10 +1490,10 @@ issue_4294: {
     expect: {
         A = "PASS";
         (function() {
-            var a = function({
+            var b = function({
-                [a]: {},
+                [b]: {},
             }) {}({
-                [a]: 0,
+                [b]: 0,
             });
             var b = A;
             console.log(b);
@@ -1502,3 +1502,41 @@ issue_4294: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_4298: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        (function() {
+            var a = {
+                object: "PASS",
+            };
+            function f({
+                [typeof a]: b,
+            }) {
+                var a = b;
+                return a;
+            }
+            var c = f(a);
+            console.log(c);
+        })();
+    }
+    expect: {
+        (function() {
+            var a = {
+                object: "PASS",
+            };
+            function f({
+                [typeof a]: b,
+            }) {
+                var a = b;
+                return a;
+            }
+            var c = f(a);
+            console.log(c);
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}