Add support for the optional chaining operator

lib/patch-eslint-scope.js

@@ -233,6 +233,10 @@ function monkeypatch(modules) {
   // visit ClassPrivateProperty as a Property.
   referencer.prototype.ClassPrivateProperty = visitClassProperty;
 
+  // visit OptionalMemberExpression as a MemberExpression.
+  referencer.prototype.OptionalMemberExpression =
+    referencer.prototype.MemberExpression;
+
   // visit flow type in FunctionDeclaration, FunctionExpression, ArrowFunctionExpression
   var visitFunction = referencer.prototype.visitFunction;
   referencer.prototype.visitFunction = function(node) {

test/non-regression.js

@@ -1651,6 +1651,18 @@ describe("verify", () => {
     });
   });
 
+  describe("optional chaining operator", () => {
+    it("should not be undefined #595", () => {
+      verifyAndAssertMessages(
+        `
+            const foo = {};
+            foo?.bar;
+        `,
+        { "no-undef": 1 }
+      );
+    });
+  });
+
   it("flow types on class method should be visited correctly", () => {
     verifyAndAssertMessages(
       `