Fix test regression in asyncHelpers.js (#4197)

harness/asyncHelpers.js

@@ -3,7 +3,7 @@
 /*---
 description: |
     A collection of assertion and wrapper functions for testing asynchronous built-ins.
-defines: [asyncTest]
+defines: [asyncTest, assert.throwsAsync]
 ---*/
 
 /**
@@ -50,18 +50,21 @@ function asyncTest(testFunc) {
  */
 assert.throwsAsync = function (expectedErrorConstructor, func, message) {
   return new Promise(function (resolve) {
-    var expectedName = expectedErrorConstructor.name;
-    var expectation = "Expected a " + expectedName + " to be thrown asynchronously";
     var fail = function (detail) {
       if (message === undefined) {
         throw new Test262Error(detail);
       }
       throw new Test262Error(message + " " + detail);
     };
-    var res;
+    if (typeof expectedErrorConstructor !== "function") {
+      fail("assert.throwsAsync called with an argument that is not an error constructor");
+    }
     if (typeof func !== "function") {
       fail("assert.throwsAsync called with an argument that is not a function");
     }
+    var expectedName = expectedErrorConstructor.name;
+    var expectation = "Expected a " + expectedName + " to be thrown asynchronously";
+    var res;
     try {
       res = func();
     } catch (thrown) {
@@ -70,9 +73,17 @@ assert.throwsAsync = function (expectedErrorConstructor, func, message) {
     if (res === null || typeof res !== "object" || typeof res.then !== "function") {
       fail(expectation + " but result was not a thenable");
     }
-
+    var onResFulfilled, onResRejected;
+    var resSettlementP = new Promise(function (onFulfilled, onRejected) {
+      onResFulfilled = onFulfilled;
+      onResRejected = onRejected;
+    });
     try {
-      resolve(Promise.resolve(res).then(
+      res.then(onResFulfilled, onResRejected)
+    } catch (thrown) {
+      fail(expectation + " but .then threw synchronously");
+    }
+    resolve(resSettlementP.then(
       function () {
         fail(expectation + " but no exception was thrown at all");
       },
@@ -90,8 +101,5 @@ assert.throwsAsync = function (expectedErrorConstructor, func, message) {
         }
       }
     ));
-    } catch (thrown) {
-      fail(expectation + " but .then threw synchronously");
-    }
   });
 };

test/harness/asyncHelpers-throwsAsync-func-never-settles.js

@@ -0,0 +1,37 @@
+// Copyright (C) 2024 Julián Espina. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+description: |
+    assert.throwsAsync returns a promise that never settles if func returns a thenable that never settles.
+flags: [async]
+includes: [asyncHelpers.js]
+---*/
+
+var realDone = $DONE;
+var doneCalls = 0
+globalThis.$DONE = function () {
+  doneCalls++;
+}
+
+function delay() {
+  var later = Promise.resolve();
+  for (var i = 0; i < 100; i++) {
+    later = later.then();
+  }
+  return later;
+}
+
+(async function () {
+  // Spy on the promise returned by an invocation of assert.throwsAsync
+  // with a function that returns a thenable which never settles.
+  var neverSettlingThenable = { then: function () { } };
+  const p = assert.throwsAsync(TypeError, function () { return neverSettlingThenable });
+  assert(p instanceof Promise, "assert.throwsAsync should return a promise");
+  p.then($DONE, $DONE);
+})()
+  // Give it a long time to try.
+  .then(delay, delay)
+  .then(function () {
+    assert.sameValue(doneCalls, 0, "$DONE should not have been called")
+  })
+  .then(realDone, realDone);