[promise] separate PerformPromiseThen from PromiseThen

test/mjsunit/harmony/async-await-species.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-async-await --allow-natives-syntax
+
+function assertEqualsAsync(expected, run, msg) {
+  var actual;
+  var hadValue = false;
+  var hadError = false;
+  var promise = run();
+
+  if (typeof promise !== "object" || typeof promise.then !== "function") {
+    throw new MjsUnitAssertionError(
+        "Expected " + run.toString() +
+        " to return a Promise, but it returned " + PrettyPrint(promise));
+  }
+
+  promise.then(function(value) { hadValue = true; actual = value; },
+               function(error) { hadError = true; actual = error; });
+
+  assertFalse(hadValue || hadError);
+
+  %RunMicrotasks();
+
+  if (hadError) throw actual;
+
+  assertTrue(
+      hadValue, "Expected '" + run.toString() + "' to produce a value");
+
+  assertEquals(expected, actual, msg);
+};
+
+function getStack(error) {
+  var stack = error.stack.split('\n').
+    filter(function(line) {
+        return /^\s*at [a-zA-Z0-9_]/.test(line);
+    }).
+    map(line => line.replace(/^\s*at ([a-zA-Z0-9_\.\[\]]+).*/, "$1"));
+
+  // remove `Promise.then()` invocation by assertEqualsAsync()
+  if (stack[2] === "assertEqualsAsync") return [];
+
+  return stack.reverse();
+}
+
+var log = [];
+class FakePromise extends Promise {
+  constructor(executor) {
+    var stack = getStack(new Error("Getting Callstack"));
+    if (stack.length) {
+      while (stack[0] === "assertEqualsAsync") stack.shift();
+      log.push("@@Species: [" + stack.join(" > ") + "]");
+    }
+    return new Promise(executor);
+  }
+};
+
+Object.defineProperty(Promise, Symbol.species, {
+  value: FakePromise,
+  configurable: true,
+  enumerable: false,
+  writable: false
+});
+
+// Internal `AsyncFunctionAwait` only --- no @@species invocations.
+async function test() { return await "foo"; }
+assertEqualsAsync("foo", function testInternalOnly() { return test(); },
+                  "should not call Promise[@@Species]");
+assertEquals([], log);
+
+log.length = 0;
+assertEqualsAsync(
+    "foo",
+    function testThenOnReturnedPromise() {
+      return test().then(x => (log.push("Then: " + x), x));
+    },
+    "should call Promise[@@Species] after non-internal Then");
+assertEquals([
+  "@@Species: [testThenOnReturnedPromise > Promise.then > FakePromise]",
+  "Then: foo"
+], log);