Use global Iterator.prototype for down-level generators (#59514)

src/testRunner/unittests/evaluation/asyncGenerator.ts

@@ -83,4 +83,26 @@ describe("unittests:: evaluation:: asyncGeneratorEvaluation", () => {
             { done: true, value: 2 },
         ]);
     });
+    it("Supports global `AsyncIterator.prototype` if present", () => {
+        class AsyncIterator {}
+        const { gen } = evaluator.evaluateTypeScript(
+            `
+            export async function * gen() {}
+            `,
+            { target: ts.ScriptTarget.ES5 },
+            { AsyncIterator },
+        );
+        const g = gen();
+        assert.instanceOf(g, AsyncIterator);
+    });
+    it("Ignores global `AsyncIterator.prototype` if missing", () => {
+        const { gen } = evaluator.evaluateTypeScript(
+            `
+            export async function * gen() {}
+            `,
+            { target: ts.ScriptTarget.ES5 },
+            { AsyncIterator: undefined },
+        );
+        gen();
+    });
 });

src/testRunner/unittests/evaluation/generator.ts

@@ -39,4 +39,26 @@ describe("unittests:: evaluation:: generatorEvaluation", () => {
         assert.deepEqual(g.next(), { value: undefined, done: true });
         assert.deepEqual(output, []);
     });
+    it("Supports global `Iterator.prototype` if present", () => {
+        class Iterator {}
+        const { gen } = evaluator.evaluateTypeScript(
+            `
+            export function * gen() {}
+            `,
+            { target: ts.ScriptTarget.ES5 },
+            { Iterator },
+        );
+        const g = gen();
+        assert.instanceOf(g, Iterator);
+    });
+    it("Ignores global `Iterator.prototype` if missing", () => {
+        const { gen } = evaluator.evaluateTypeScript(
+            `
+            export function * gen() {}
+            `,
+            { target: ts.ScriptTarget.ES5 },
+            { Iterator: undefined },
+        );
+        gen();
+    });
 });