Allow undefined rejection reason

While not recommended because it makes tracking errors more difficult, it's now possible to reject a promise without explicit reason, in which case, a built-in `QPromiseUndefinedException` is thrown. This is done in anticipation of handling rejection signals without argument.

tests/auto/qtpromise/qpromise/construct/tst_construct.cpp

@@ -33,6 +33,8 @@ private Q_SLOTS:
     void rejectSync_void();
     void rejectAsync();
     void rejectAsync_void();
+    void rejectUndefined();
+    void rejectUndefined_void();
     void connectAndResolve();
     void connectAndReject();
 };
@@ -234,6 +236,30 @@ void tst_qpromise_construct::rejectThrowTwoArgs_void()
     QCOMPARE(waitForError(p, QString()), QString("foo"));
 }
 
+void tst_qpromise_construct::rejectUndefined()
+{
+    QPromise<int> p([](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+        QtPromisePrivate::qtpromise_defer([=]() {
+            reject();
+        });
+    });
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForRejected<QPromiseUndefinedException>(p), true);
+}
+
+void tst_qpromise_construct::rejectUndefined_void()
+{
+    QPromise<void> p([](const QPromiseResolve<void>&, const QPromiseReject<void>& reject) {
+        QtPromisePrivate::qtpromise_defer([=]() {
+            reject();
+        });
+    });
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForRejected<QPromiseUndefinedException>(p), true);
+}
+
 // https://github.com/simonbrunel/qtpromise/issues/6
 void tst_qpromise_construct::connectAndResolve()
 {

tests/auto/qtpromise/shared/utils.h

@@ -44,4 +44,14 @@ static inline E waitForError(const QtPromise::QPromise<void>& promise, const E&
     return error;
 }
 
+template <typename E, typename T>
+static inline bool waitForRejected(const T& promise)
+{
+    bool result = false;
+    promise.tapFail([&](const E&) {
+        result = true;
+    }).wait();
+    return result;
+}
+
 #endif // QTPROMISE_TESTS_AUTO_SHARED_UTILS_H