Long live QtFuture::makeReadyVoidFuture() and QtFuture::makeReadyValueFuture()

[ChangeLog][QtCore][QFuture] Added QtFuture::makeReadyVoidFuture()
and QtFuture::makeReadyValueFuture().

Basically, these methods behave like QtFuture::makeReadyFuture(), but
QtFuture::makeReadyValueFuture() does not have a "const QList<T> &"
specialization returning QFuture<T> instead of QFuture<QList<T>>,
which allows it to always behave consistently.

This patch also introduces usage of the new methods around qtbase.

Task-number: QTBUG-109677
Change-Id: I89df8b26d82c192baad69efb5df517a8b182995f
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

8 files changed, 147 insertions, 50 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
index 494719d988..ea104ec1a9 100644
--- a/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp
@@ -239,9 +239,9 @@ auto future = QtConcurrent::run([] {
 
 //! [20]
 QObject *context = ...;
-auto future = cachedResultsReady ? QtFuture::makeReadyFuture(results)
-                                 : QtConcurrent::run([] { /* compute results */});
-auto continuation = future.then(context, [] (Results results) {
+auto future = cachedResultsReady ? QtFuture::makeReadyValueFuture(result)
+                                 : QtConcurrent::run([] { /* compute result */});
+auto continuation = future.then(context, [] (Result result) {
     // Runs in the context's thread
 }).then([] {
     // May or may not run in the context's thread
@@ -413,3 +413,17 @@ auto f = QtFuture::makeReadyRangeFuture({1, 2, 3});
 const int count = f.resultCount(); // count == 3
 const auto results = f.results(); // results == { 1, 2, 3 }
 //! [34]
+
+//! [35]
+auto f = QtFuture::makeReadyValueFuture(std::make_unique<int>(42));
+...
+const int result = *f.takeResult(); // result == 42
+//! [35]
+
+//! [36]
+auto f = QtFuture::makeReadyVoidFuture();
+...
+const bool started = f.isStarted(); // started == true
+const bool running = f.isRunning(); // running == false
+const bool finished = f.isFinished(); // finished == true
+//! [36]
diff --git a/src/corelib/platform/android/qandroidextras.cpp b/src/corelib/platform/android/qandroidextras.cpp
index b7c07a8f03..85f8a7e808 100644
--- a/src/corelib/platform/android/qandroidextras.cpp
+++ b/src/corelib/platform/android/qandroidextras.cpp
@@ -1095,7 +1095,7 @@ requestPermissionsInternal(const QStringList &permissions)
     }
 
     if (!QtAndroidPrivate::acquireAndroidDeadlockProtector())
-        return QtFuture::makeReadyFuture(QtAndroidPrivate::Denied);
+        return QtFuture::makeReadyValueFuture(QtAndroidPrivate::Denied);
 
     QSharedPointer<QPromise<QtAndroidPrivate::PermissionResult>> promise;
     promise.reset(new QPromise<QtAndroidPrivate::PermissionResult>());
@@ -1145,7 +1145,7 @@ QtAndroidPrivate::requestPermissions(const QStringList &permissions)
 {
     // avoid the uneccessary call and response to an empty permission string
     if (permissions.isEmpty())
-        return QtFuture::makeReadyFuture(QtAndroidPrivate::Denied);
+        return QtFuture::makeReadyValueFuture(QtAndroidPrivate::Denied);
     return requestPermissionsInternal(permissions);
 }
 
@@ -1168,7 +1168,7 @@ QtAndroidPrivate::checkPermission(const QString &permission)
                                                       QJniObject::fromString(permission).object());
         result = resultFromAndroid(res);
     }
-    return QtFuture::makeReadyFuture(result);
+    return QtFuture::makeReadyValueFuture(result);
 }
 
 bool QtAndroidPrivate::registerPermissionNatives()
diff --git a/src/corelib/thread/qfuture.h b/src/corelib/thread/qfuture.h
index 34fb8a039d..219d3368f0 100644
--- a/src/corelib/thread/qfuture.h
+++ b/src/corelib/thread/qfuture.h
@@ -522,6 +522,16 @@ QFuture<std::variant<std::decay_t<Futures>...>> whenAny(Futures &&... futures);
 
 #endif // Q_QDOC
 
+#if defined(Q_QDOC)
+static QFuture<void> makeReadyFuture()
+#else
+template<typename T = void>
+static QFuture<T> makeReadyFuture()
+#endif
+{
+    return makeReadyVoidFuture();
+}
+
 } // namespace QtFuture
 
 Q_DECLARE_SEQUENTIAL_ITERATOR(Future)
diff --git a/src/corelib/thread/qfuture.qdoc b/src/corelib/thread/qfuture.qdoc
index 7ba4debeea..e9aabdda2d 100644
--- a/src/corelib/thread/qfuture.qdoc
+++ b/src/corelib/thread/qfuture.qdoc
@@ -118,15 +118,16 @@
     combine several futures and track when the last or first of them completes.
 
     A ready QFuture object with a value or a QFuture object holding exception can
-    be created using convenience functions QtFuture::makeReadyFuture(),
-    QtFuture::makeReadyRangeFuture(), and QtFuture::makeExceptionalFuture().
+    be created using convenience functions QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(), and
+    QtFuture::makeExceptionalFuture().
 
     \note To start a computation and store results in a QFuture, use QPromise or
     one of the APIs in the \l {Qt Concurrent} framework.
 
-    \sa QPromise, QtFuture::connect(), QtFuture::makeReadyFuture(),
-    QtFuture::makeReadyRangeFuture(), QtFuture::makeExceptionalFuture(),
-    QFutureWatcher, {Qt Concurrent}
+    \sa QPromise, QtFuture::connect(), QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(),
+    QtFuture::makeExceptionalFuture(), QFutureWatcher, {Qt Concurrent}
 */
 
 /*! \fn template <typename T> QFuture<T>::QFuture()
@@ -982,7 +983,9 @@
     const int result = *f.takeResult(); // result == 42
     \endcode
 
-    \sa QFuture, QtFuture::makeExceptionalFuture()
+    \sa QFuture, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(),
+    QtFuture::makeExceptionalFuture()
 */
 
 /*! \fn QFuture<void> QtFuture::makeReadyFuture()
@@ -1003,7 +1006,9 @@
     \endcode
 
     \sa QFuture, QFuture::isStarted(), QFuture::isRunning(),
-    QFuture::isFinished(), QtFuture::makeExceptionalFuture()
+    QFuture::isFinished(), QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(),
+    QtFuture::makeExceptionalFuture()
 */
 
 /*! \fn template<typename T> static QFuture<T> QtFuture::makeReadyFuture(const QList<T> &values)
@@ -1021,7 +1026,38 @@
     const auto results = f.results(); // results == { 1, 2, 3 }
     \endcode
 
-    \sa QFuture, QtFuture::makeExceptionalFuture()
+    \sa QFuture, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(),
+    QtFuture::makeExceptionalFuture()
+*/
+
+/*! \fn template<typename T> static QFuture<std::decay_t<T>> QtFuture::makeReadyValueFuture(T &&value)
+
+    \since 6.6
+
+    Creates and returns a QFuture which already has a result \a value.
+    The returned QFuture has a type of std::decay_t<T>, where T is not void.
+    The returned QFuture will already be in the finished state.
+
+    \snippet code/src_corelib_thread_qfuture.cpp 35
+
+    \sa QFuture, QtFuture::makeReadyRangeFuture(),
+    QtFuture::makeReadyVoidFuture(), QtFuture::makeExceptionalFuture()
+*/
+
+/*! \fn QFuture<void> QtFuture::makeReadyVoidFuture()
+
+    \since 6.6
+
+    Creates and returns a void QFuture. Such QFuture can't store any result.
+    One can use it to query the state of the computation.
+    The returned QFuture will already be in the finished state.
+
+    \snippet code/src_corelib_thread_qfuture.cpp 36
+
+    \sa QFuture, QFuture::isStarted(), QFuture::isRunning(),
+    QFuture::isFinished(), QtFuture::makeReadyValueFuture(),
+    QtFuture::makeReadyRangeFuture(), QtFuture::makeExceptionalFuture()
 */
 
 /*! \fn template<typename T> static QFuture<T> QtFuture::makeExceptionalFuture(const QException &exception)
@@ -1041,7 +1077,8 @@
     }
     \endcode
 
-    \sa QFuture, QException, QtFuture::makeReadyFuture()
+    \sa QFuture, QException, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture()
 */
 
 /*! \fn template<typename T> static QFuture<T> QtFuture::makeExceptionalFuture(std::exception_ptr exception)
@@ -1066,7 +1103,8 @@
     }
     \endcode
 
-    \sa QFuture, QException, QtFuture::makeReadyFuture()
+    \sa QFuture, QException, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture()
 */
 
 /*! \fn template<typename Container, QtFuture::if_container_with_input_iterators<Container>> static QFuture<QtFuture::ContainedType<Container>> QtFuture::makeReadyRangeFuture(Container &&container)
@@ -1085,7 +1123,8 @@
     \dots
     \snippet code/src_corelib_thread_qfuture.cpp 34
 
-    \sa QFuture, QtFuture::makeReadyFuture(), QtFuture::makeExceptionalFuture()
+    \sa QFuture, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeExceptionalFuture()
 */
 
 /*! \fn template<typename ValueType> static QFuture<ValueType> QtFuture::makeReadyRangeFuture(std::initializer_list<ValueType> values)
@@ -1100,7 +1139,8 @@
     \dots
     \snippet code/src_corelib_thread_qfuture.cpp 34
 
-    \sa QFuture, QtFuture::makeReadyFuture(), QtFuture::makeExceptionalFuture()
+    \sa QFuture, QtFuture::makeReadyVoidFuture(),
+    QtFuture::makeReadyValueFuture(), QtFuture::makeExceptionalFuture()
 */
 
 /*! \fn template<class T> template<class Function> QFuture<typename QFuture<T>::ResultType<Function>> QFuture<T>::then(Function &&function)
diff --git a/src/corelib/thread/qfuture_impl.h b/src/corelib/thread/qfuture_impl.h
index 5ed7a4d27f..b282903405 100644
--- a/src/corelib/thread/qfuture_impl.h
+++ b/src/corelib/thread/qfuture_impl.h
@@ -996,8 +996,8 @@ static QFuture<ValueType> makeReadyRangeFuture(std::initializer_list<ValueType>
     return QtPrivate::makeReadyRangeFutureImpl(QList<ValueType>{values});
 }
 
-template<typename T, typename = QtPrivate::EnableForNonVoid<T>>
-static QFuture<std::decay_t<T>> makeReadyFuture(T &&value)
+template<typename T>
+static QFuture<std::decay_t<T>> makeReadyValueFuture(T &&value)
 {
     QFutureInterface<std::decay_t<T>> promise;
     promise.reportStarted();
@@ -1007,20 +1007,17 @@ static QFuture<std::decay_t<T>> makeReadyFuture(T &&value)
     return promise.future();
 }
 
-#if defined(Q_QDOC)
-static QFuture<void> makeReadyFuture()
-#else
-template<typename T = void>
-static QFuture<T> makeReadyFuture()
-#endif
-{
-    QFutureInterface<T> promise;
-    promise.reportStarted();
-    promise.reportFinished();
+Q_CORE_EXPORT QFuture<void> makeReadyVoidFuture(); // implemented in qfutureinterface.cpp
 
-    return promise.future();
+template<typename T, typename = QtPrivate::EnableForNonVoid<T>>
+static QFuture<std::decay_t<T>> makeReadyFuture(T &&value)
+{
+    return makeReadyValueFuture(std::forward<T>(value));
 }
 
+// the void specialization is moved to the end of qfuture.h, because it now
+// uses makeReadyVoidFuture() and required QFuture<void> to be defined.
+
 template<typename T>
 static QFuture<T> makeReadyFuture(const QList<T> &values)
 {
@@ -1127,7 +1124,7 @@ QFuture<OutputSequence> whenAllImpl(InputIt first, InputIt last)
 {
     const qsizetype size = std::distance(first, last);
     if (size == 0)
-        return QtFuture::makeReadyFuture(OutputSequence());
+        return QtFuture::makeReadyValueFuture(OutputSequence());
 
     const auto context = std::make_shared<QtPrivate::WhenAllContext<OutputSequence>>(size);
     context->futures.resize(size);
@@ -1166,7 +1163,7 @@ QFuture<QtFuture::WhenAnyResult<typename Future<ValueType>::type>> whenAnyImpl(I
 
     const qsizetype size = std::distance(first, last);
     if (size == 0) {
-        return QtFuture::makeReadyFuture(
+        return QtFuture::makeReadyValueFuture(
                 QtFuture::WhenAnyResult { qsizetype(-1), QFuture<PackagedType>() });
     }
 
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index e656527e04..790963547c 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -889,4 +889,17 @@ bool QFutureInterfaceBase::launchAsync() const
     return d->launchAsync;
 }
 
+namespace QtFuture {
+
+QFuture<void> makeReadyVoidFuture()
+{
+    QFutureInterface<void> promise;
+    promise.reportStarted();
+    promise.reportFinished();
+
+    return promise.future();
+}
+
+} // namespace QtFuture
+
 QT_END_NAMESPACE
diff --git a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
index f457ba4668..bb78898836 100644
--- a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
+++ b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
@@ -3052,7 +3052,7 @@ void tst_QFuture::cancelContinuations()
 
     // The chain is cancelled before the execution of continuations
     {
-        auto f = QtFuture::makeReadyFuture(42);
+        auto f = QtFuture::makeReadyValueFuture(42);
         f.cancel();
 
         int checkpoint = 0;
@@ -3301,7 +3301,8 @@ void tst_QFuture::continuationsWithMoveOnlyLambda()
     // .then()
     {
         std::unique_ptr<int> uniquePtr(new int(42));
-        auto future = QtFuture::makeReadyFuture().then([p = std::move(uniquePtr)] { return *p; });
+        auto future = QtFuture::makeReadyVoidFuture()
+                .then([p = std::move(uniquePtr)] { return *p; });
         QCOMPARE(future.result(), 42);
     }
     // .then() with thread pool
@@ -3309,8 +3310,8 @@ void tst_QFuture::continuationsWithMoveOnlyLambda()
         QThreadPool pool;
 
         std::unique_ptr<int> uniquePtr(new int(42));
-        auto future =
-                QtFuture::makeReadyFuture().then(&pool, [p = std::move(uniquePtr)] { return *p; });
+        auto future = QtFuture::makeReadyVoidFuture()
+                .then(&pool, [p = std::move(uniquePtr)] { return *p; });
         QCOMPARE(future.result(), 42);
     }
     // .then() with context
@@ -3318,8 +3319,8 @@ void tst_QFuture::continuationsWithMoveOnlyLambda()
         QObject object;
 
         std::unique_ptr<int> uniquePtr(new int(42));
-        auto future = QtFuture::makeReadyFuture().then(&object,
-                                                       [p = std::move(uniquePtr)] { return *p; });
+        auto future = QtFuture::makeReadyVoidFuture()
+                .then(&object, [p = std::move(uniquePtr)] { return *p; });
         QCOMPARE(future.result(), 42);
     }
 
@@ -4113,6 +4114,28 @@ void tst_QFuture::createReadyFutures()
         QCOMPARE(f.results(), values);
     }
 
+    // test makeReadyValueFuture<T>()
+    {
+        const int val = 42;
+        auto f = QtFuture::makeReadyValueFuture(val);
+        QCOMPARE_EQ(f.result(), val);
+
+        int otherVal = 42;
+        f = QtFuture::makeReadyValueFuture(otherVal);
+        QCOMPARE_EQ(f.result(), otherVal);
+    }
+    {
+        auto f = QtFuture::makeReadyValueFuture(std::make_unique<int>(42));
+        QCOMPARE(*f.takeResult(), 42);
+    }
+    // test makeReadyVoidFuture()
+    {
+        auto f = QtFuture::makeReadyVoidFuture();
+        QVERIFY(f.isStarted());
+        QVERIFY(!f.isRunning());
+        QVERIFY(f.isFinished());
+    }
+
 #ifndef QT_NO_EXCEPTIONS
     // using QException
     {
@@ -4236,7 +4259,7 @@ void tst_QFuture::createReadyFutures()
 void tst_QFuture::getFutureInterface()
 {
     const int val = 42;
-    QFuture<int> f = QtFuture::makeReadyFuture(val);
+    QFuture<int> f = QtFuture::makeReadyValueFuture(val);
 
     auto interface = QFutureInterfaceBase::get(f);
     QCOMPARE(interface.resultCount(), 1);
@@ -4250,7 +4273,7 @@ void tst_QFuture::convertQMetaType()
     QVERIFY(QMetaType::canConvert(intType, voidType));
 
     const int val = 42;
-    QFuture<int> f = QtFuture::makeReadyFuture(val);
+    QFuture<int> f = QtFuture::makeReadyValueFuture(val);
     auto variant = QVariant::fromValue(f);
     QVERIFY(variant.convert(voidType));
 
diff --git a/tests/benchmarks/corelib/thread/qfuture/tst_bench_qfuture.cpp b/tests/benchmarks/corelib/thread/qfuture/tst_bench_qfuture.cpp
index 7c0e3b2fcf..74dd549462 100644
--- a/tests/benchmarks/corelib/thread/qfuture/tst_bench_qfuture.cpp
+++ b/tests/benchmarks/corelib/thread/qfuture/tst_bench_qfuture.cpp
@@ -13,7 +13,7 @@ class tst_QFuture : public QObject
     Q_OBJECT
 
 private slots:
-    void makeReadyfuture();
+    void makeReadyValueFuture();
 #ifndef QT_NO_EXCEPTIONS
     void makeExceptionalFuture();
 #endif
@@ -43,10 +43,10 @@ private slots:
     void progressText();
 };
 
-void tst_QFuture::makeReadyfuture()
+void tst_QFuture::makeReadyValueFuture()
 {
     QBENCHMARK {
-        auto future = QtFuture::makeReadyFuture(42);
+        auto future = QtFuture::makeReadyValueFuture(42);
         Q_UNUSED(future);
     }
 }
@@ -64,7 +64,7 @@ void tst_QFuture::makeExceptionalFuture()
 
 void tst_QFuture::result()
 {
-    auto future = QtFuture::makeReadyFuture(42);
+    auto future = QtFuture::makeReadyValueFuture(42);
 
     QBENCHMARK {
         auto value = future.result();
@@ -92,7 +92,7 @@ void tst_QFuture::results()
 void tst_QFuture::takeResult()
 {
     QBENCHMARK {
-        auto future = QtFuture::makeReadyFuture(42);
+        auto future = QtFuture::makeReadyValueFuture(42);
         auto value = future.takeResult();
         Q_UNUSED(value);
     }
@@ -140,7 +140,7 @@ void tst_QFuture::reportException()
 
 void tst_QFuture::then()
 {
-    auto f = QtFuture::makeReadyFuture(42);
+    auto f = QtFuture::makeReadyValueFuture(42);
     QBENCHMARK {
         auto future = f.then([](int value) { return value; });
         Q_UNUSED(future);
@@ -149,7 +149,7 @@ void tst_QFuture::then()
 
 void tst_QFuture::thenVoid()
 {
-    auto f = QtFuture::makeReadyFuture();
+    auto f = QtFuture::makeReadyVoidFuture();
     QBENCHMARK {
         auto future = f.then([] {});
         Q_UNUSED(future);
@@ -205,7 +205,7 @@ void tst_QFuture::onFailedVoid()
 
 void tst_QFuture::thenOnFailed()
 {
-    auto f = QtFuture::makeReadyFuture(42);
+    auto f = QtFuture::makeReadyValueFuture(42);
     QBENCHMARK {
         auto future =
                 f.then([](int) { throw std::runtime_error("error"); }).onFailed([] { return 0; });
@@ -215,7 +215,7 @@ void tst_QFuture::thenOnFailed()
 
 void tst_QFuture::thenOnFailedVoid()
 {
-    auto f = QtFuture::makeReadyFuture();
+    auto f = QtFuture::makeReadyVoidFuture();
     QBENCHMARK {
         auto future = f.then([] { throw std::runtime_error("error"); }).onFailed([] {});
         Q_UNUSED(future);