Fix test regression in asyncHelpers.js

harness/asyncHelpers.js

@@ -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 +73,9 @@ 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 thenable;
     try {
-      resolve(Promise.resolve(res).then(
+      thenable = res.then(
         function () {
           fail(expectation + " but no exception was thrown at all");
         },
@@ -89,9 +92,13 @@ assert.throwsAsync = function (expectedErrorConstructor, func, message) {
             fail(expectation + " but got a " + actualName);
           }
         }
-      ));
+      );
     } catch (thrown) {
       fail(expectation + " but .then threw synchronously");
     }
+    if (thenable === null || typeof thenable !== "object" || typeof thenable.then !== "function") {
+      fail(expectation + " but result was not a conforming thenable");
+    }
+    resolve(Promise.resolve(thenable));
   });
 };

test/harness/asyncHelpers-throwsAsync-func-bad-thenable.js

@@ -0,0 +1,34 @@
+// 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 rejects if func returns a bad thenable.
+flags: [async]
+includes: [asyncHelpers.js]
+---*/
+
+asyncTest(async function() {
+  var caught = false;
+  var func = function () {
+    return {
+      then: function () {},
+    };
+  };
+  const p = assert.throwsAsync(TypeError, func);
+  assert(p instanceof Promise, "assert.throwsAsync should return a promise");
+  try {
+    await p;
+  } catch (e) {
+    caught = true;
+    assert.sameValue(
+      e.constructor,
+      Test262Error,
+      "throwsAsync should reject bad thenable with a Test262Error"
+    );
+  } finally {
+    assert(
+      caught,
+      "assert.throwsAsync did not reject bad thenable in " + func
+    );
+  }
+});