Require .next method on delegate iterator (#630)

packages/runtime/runtime.js

@@ -329,31 +329,32 @@ var runtime = (function (exports) {
   // delegate iterator, or by modifying context.method and context.arg,
   // setting context.delegate to null, and returning the ContinueSentinel.
   function maybeInvokeDelegate(delegate, context) {
-    var method = delegate.iterator[context.method];
+    var methodName = context.method;
+    var method = delegate.iterator[methodName];
     if (method === undefined) {
       // A .throw or .return when the delegate iterator has no .throw
-      // method always terminates the yield* loop.
+      // method, or a missing .next mehtod, always terminate the
+      // yield* loop.
       context.delegate = null;
 
-      if (context.method === "throw") {
-        // Note: ["return"] must be used for ES3 parsing compatibility.
-        if (delegate.iterator["return"]) {
-          // If the delegate iterator has a return method, give it a
-          // chance to clean up.
-          context.method = "return";
-          context.arg = undefined;
-          maybeInvokeDelegate(delegate, context);
+      // Note: ["return"] must be used for ES3 parsing compatibility.
+      if (methodName === "throw" && delegate.iterator["return"]) {
+        // If the delegate iterator has a return method, give it a
+        // chance to clean up.
+        context.method = "return";
+        context.arg = undefined;
+        maybeInvokeDelegate(delegate, context);
 
-          if (context.method === "throw") {
-            // If maybeInvokeDelegate(context) changed context.method from
-            // "return" to "throw", let that override the TypeError below.
-            return ContinueSentinel;
-          }
+        if (context.method === "throw") {
+          // If maybeInvokeDelegate(context) changed context.method from
+          // "return" to "throw", let that override the TypeError below.
+          return ContinueSentinel;
         }
-
+      }
+      if (methodName !== "return") {
         context.method = "throw";
         context.arg = new TypeError(
-          "The iterator does not provide a 'throw' method");
+          "The iterator does not provide a '" + methodName + "' method");
       }
 
       return ContinueSentinel;

test/tests.es6.js

@@ -1503,6 +1503,18 @@ describe("delegated yield", function() {
 
     check(gen(iterator), [], "1foo");
   });
+
+  it("should throw if the delegated iterable's iterator doesn't have .next", function() {
+    var it = function* () {
+      yield* {
+        [Symbol.iterator]: x => [],
+      };
+    }();
+
+    assert.throws(() => {
+      it.next();
+    }, TypeError);
+  });
 });
 
 (fullCompatibility