Forbid implicit conversions between QFuture and other types

- Remove the casting operator of QFuture<T> to T. It calls
  QFuture::result(), which may lead to undefined behavior if the user
  has moved the results from QFuture via QFuture::takeResult() before
  trying to do the conversion.

- Disable implicit conversion of QFuture<T> to QFuture<void>, by making
  the constructor explicit. If the users really intend to do the
  conversion, they should do it explicitly.

[ChangeLog][Source-Incompatible Changes][QFuture] Implicit conversions
of QFuture<T> to T and to QFuture<void> have been disabled. Use
QFuture::result() or QFuture::takeResult() where you need to convert
QFuture<T> to T. Use the explicit QFuture<void>(const QFuture<T> &)
constructor to convert QFuture<T> to QFuture<void>.

Change-Id: I153d4137d36365b1611ac934fb3ac2eb667fdd6c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

9 files changed, 73 insertions, 87 deletions

diff --git a/examples/qtconcurrent/wordcount/main.cpp b/examples/qtconcurrent/wordcount/main.cpp
index ae8542a761..f4de2886a8 100644
--- a/examples/qtconcurrent/wordcount/main.cpp
+++ b/examples/qtconcurrent/wordcount/main.cpp
@@ -157,7 +157,7 @@ int main(int argc, char** argv)
     {
         QElapsedTimer timer;
         timer.start();
-        WordCount total = mappedReduced(files, countWords, reduce);
+        WordCount total = mappedReduced(files, countWords, reduce).result();
         mapReduceTime = timer.elapsed();
         qDebug() << "MapReduce" << mapReduceTime;
     }
diff --git a/src/corelib/thread/qfuture.h b/src/corelib/thread/qfuture.h
index c5966f7893..44c5331261 100644
--- a/src/corelib/thread/qfuture.h
+++ b/src/corelib/thread/qfuture.h
@@ -81,7 +81,7 @@ public:
     }
 
     template<typename U, typename V = T, typename = QtPrivate::EnableForVoid<V>>
-    QFuture(const QFuture<U> &other) : d(other.d)
+    explicit QFuture(const QFuture<U> &other) : d(other.d)
     {
     }
 
@@ -96,9 +96,6 @@ public:
     ~QFuture() { }
     QFuture(const QFuture<T> &) { }
     QFuture<T> & operator=(const QFuture<T> &) { }
-
-    // This is required to allow QDoc to find the declaration of operator T().
-    operator T() const;
 #endif
 
     void cancel() { d.cancel(); }
@@ -150,10 +147,6 @@ QT_WARNING_POP
     template<typename U = T, typename = QtPrivate::EnableForNonVoid<U>>
     bool isResultReadyAt(int resultIndex) const { return d.isResultReadyAt(resultIndex); }
 
-    // operator T()
-    template<typename U = T>
-    operator typename std::enable_if_t<!std::is_same_v<U, void>, U>() const { return result(); }
-
     template<typename U = T, typename = QtPrivate::EnableForNonVoid<U>>
     QList<T> results() const { return d.results(); }
 
diff --git a/src/corelib/thread/qfuture.qdoc b/src/corelib/thread/qfuture.qdoc
index 59f80da4af..82b7657016 100644
--- a/src/corelib/thread/qfuture.qdoc
+++ b/src/corelib/thread/qfuture.qdoc
@@ -433,19 +433,6 @@
     \sa resultAt(), resultCount(), takeResult()
 */
 
-/*! \fn template <typename T> QFuture<T>::operator T() const
-
-    Returns the first result in the future. If the result is not immediately
-    available, this function will block and wait for the result to become
-    available. This is a convenience method for calling result() or
-    resultAt(0).
-
-    \note Calling this function leads to undefined behavior if isValid()
-    returns \c false for this QFuture.
-
-    \sa result(), resultAt(), results(), takeResult(), isValid()
-*/
-
 /*! \fn template <typename T> QList<T> QFuture<T>::results() const
 
     Returns all results from the future. If the results are not immediately available,
diff --git a/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp b/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp
index 32023f0a66..8cce82defd 100644
--- a/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp
+++ b/tests/auto/concurrent/qtconcurrentfilter/tst_qtconcurrentfilter.cpp
@@ -333,12 +333,12 @@ void testFilteredReduced(const QList<SourceObject> &sourceObjectList,
                          ReduceObject reduceObject)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced<ResultObject>(
-                sourceObjectList, filterObject, reduceObject);
+                                         sourceObjectList, filterObject, reduceObject).result();
     QCOMPARE(result1, expectedResult);
 
     const ResultObject result2 = QtConcurrent::filteredReduced<ResultObject>(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                filterObject, reduceObject);
+                                         sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                         filterObject, reduceObject).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingFilteredReduced<ResultObject>(
@@ -362,12 +362,13 @@ void testFilteredReduced(const QList<SourceObject> &sourceObjectList,
                          QtConcurrent::ReduceOptions options)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced(
-                sourceObjectList, filterObject, reduceObject, options);
+                sourceObjectList, filterObject, reduceObject, options).result();
     QCOMPARE(result1, expectedResult);
 
-    const ResultObject result2 = QtConcurrent::filteredReduced(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), filterObject,
-                reduceObject, options);
+    const ResultObject result2 =
+            QtConcurrent::filteredReduced(sourceObjectList.constBegin(),
+                                          sourceObjectList.constEnd(),
+                                          filterObject, reduceObject, options).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingFilteredReduced(
@@ -492,13 +493,14 @@ void testFilteredReducedThreadPool(QThreadPool *pool,
                                    ReduceObject reduceObject)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced<ResultObject>(
-                pool, sourceObjectList, filterObject, reduceObject);
+                pool, sourceObjectList, filterObject, reduceObject).result();
     QCOMPARE(result1, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
-    const ResultObject result2 = QtConcurrent::filteredReduced<ResultObject>(
-                pool, sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                filterObject, reduceObject);
+    const ResultObject result2 =
+            QtConcurrent::filteredReduced<ResultObject>(pool, sourceObjectList.constBegin(),
+                                                        sourceObjectList.constEnd(), filterObject,
+                                                        reduceObject).result();
     QCOMPARE(result2, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
@@ -640,12 +642,12 @@ void testFilteredReducedInitialValue(const QList<SourceObject> &sourceObjectList
                                      InitialObject &&initialObject)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced<ResultObject>(
-                sourceObjectList, filterObject, reduceObject, initialObject);
+                sourceObjectList, filterObject, reduceObject, initialObject).result();
     QCOMPARE(result1, expectedResult);
 
     const ResultObject result2 = QtConcurrent::filteredReduced<ResultObject>(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                filterObject, reduceObject, initialObject);
+                                         sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                         filterObject, reduceObject, initialObject).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingFilteredReduced<ResultObject>(
@@ -671,12 +673,13 @@ void testFilteredReducedInitialValue(const QList<SourceObject> &sourceObjectList
                                      QtConcurrent::ReduceOptions options)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced(
-                sourceObjectList, filterObject, reduceObject, initialObject, options);
+                sourceObjectList, filterObject, reduceObject, initialObject, options).result();
     QCOMPARE(result1, expectedResult);
 
-    const ResultObject result2 = QtConcurrent::filteredReduced(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                filterObject, reduceObject, initialObject, options);
+    const ResultObject result2 =
+            QtConcurrent::filteredReduced(sourceObjectList.constBegin(),
+                                          sourceObjectList.constEnd(), filterObject, reduceObject,
+                                          initialObject, options).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingFilteredReduced(
@@ -810,13 +813,14 @@ void testFilteredReducedInitialValueThreadPool(QThreadPool *pool,
                                                InitialObject &&initialObject)
 {
     const ResultObject result1 = QtConcurrent::filteredReduced<ResultObject>(
-                pool, sourceObjectList, filterObject, reduceObject, initialObject);
+                pool, sourceObjectList, filterObject, reduceObject, initialObject).result();
     QCOMPARE(result1, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
-    const ResultObject result2 = QtConcurrent::filteredReduced<ResultObject>(
-                pool, sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                filterObject, reduceObject, initialObject);
+    const ResultObject result2 =
+            QtConcurrent::filteredReduced<ResultObject>(pool, sourceObjectList.constBegin(),
+                                                        sourceObjectList.constEnd(), filterObject,
+                                                        reduceObject, initialObject).result();
     QCOMPARE(result2, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
diff --git a/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp b/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
index ca2db64a85..e3b61bb109 100644
--- a/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
+++ b/tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp
@@ -674,11 +674,12 @@ template <typename SourceObject, typename ResultObject, typename MapObject, type
 void testMappedReduced(const QList<SourceObject> &sourceObjectList, const ResultObject &expectedResult, MapObject mapObject, ReduceObject reduceObject)
 {
     const ResultObject result1 = QtConcurrent::mappedReduced<ResultObject>(
-                sourceObjectList, mapObject, reduceObject);
+                sourceObjectList, mapObject, reduceObject).result();
     QCOMPARE(result1, expectedResult);
 
     const ResultObject result2 = QtConcurrent::mappedReduced<ResultObject>(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), mapObject, reduceObject);
+                                         sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                         mapObject, reduceObject).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingMappedReduced<ResultObject>(
@@ -694,11 +695,12 @@ template <typename SourceObject, typename ResultObject, typename MapObject, type
 void testMappedReduced(const QList<SourceObject> &sourceObjectList, const ResultObject &expectedResult, MapObject mapObject, ReduceObject reduceObject, QtConcurrent::ReduceOptions options)
 {
     const ResultObject result1 = QtConcurrent::mappedReduced(
-                sourceObjectList, mapObject, reduceObject, options);
+                sourceObjectList, mapObject, reduceObject, options).result();
     QCOMPARE(result1, expectedResult);
 
-    const ResultObject result2 = QtConcurrent::mappedReduced(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), mapObject, reduceObject, options);
+    const ResultObject result2 =
+            QtConcurrent::mappedReduced(sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                        mapObject, reduceObject, options).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingMappedReduced(
@@ -796,13 +798,15 @@ void testMappedReducedThreadPool(QThreadPool *pool,
                                  MapObject mapObject,
                                  ReduceObject reduceObject)
 {
-    const ResultObject result1 = QtConcurrent::mappedReduced<ResultObject>(pool,
-                sourceObjectList, mapObject, reduceObject);
+    const ResultObject result1 = QtConcurrent::mappedReduced<ResultObject>(
+                pool, sourceObjectList, mapObject, reduceObject).result();
     QCOMPARE(result1, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
-    const ResultObject result2 = QtConcurrent::mappedReduced<ResultObject>(pool,
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), mapObject, reduceObject);
+    const ResultObject result2 =
+            QtConcurrent::mappedReduced<ResultObject>(pool, sourceObjectList.constBegin(),
+                                                      sourceObjectList.constEnd(), mapObject,
+                                                      reduceObject).result();
     QCOMPARE(result2, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
@@ -954,12 +958,14 @@ void testMappedReducedInitialValue(const QList<SourceObject> &sourceObjectList,
                                    ReduceObject reduceObject,
                                    InitialObject &&initialObject)
 {
-    const ResultObject result1 = QtConcurrent::mappedReduced<ResultObject>(
-                sourceObjectList, mapObject, reduceObject, initialObject);
+    const ResultObject result1 =
+            QtConcurrent::mappedReduced<ResultObject>(sourceObjectList, mapObject, reduceObject,
+                                                      initialObject).result();
     QCOMPARE(result1, expectedResult);
 
     const ResultObject result2 = QtConcurrent::mappedReduced<ResultObject>(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), mapObject, reduceObject, initialObject);
+                                         sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                         mapObject, reduceObject, initialObject).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingMappedReduced<ResultObject>(
@@ -980,11 +986,12 @@ void testMappedReducedInitialValue(const QList<SourceObject> &sourceObjectList,
                                    QtConcurrent::ReduceOptions options)
 {
     const ResultObject result1 = QtConcurrent::mappedReduced(
-                sourceObjectList, mapObject, reduceObject, initialObject, options);
+                sourceObjectList, mapObject, reduceObject, initialObject, options).result();
     QCOMPARE(result1, expectedResult);
 
-    const ResultObject result2 = QtConcurrent::mappedReduced(
-                sourceObjectList.constBegin(), sourceObjectList.constEnd(), mapObject, reduceObject, initialObject, options);
+    const ResultObject result2 =
+            QtConcurrent::mappedReduced(sourceObjectList.constBegin(), sourceObjectList.constEnd(),
+                                        mapObject, reduceObject, initialObject, options).result();
     QCOMPARE(result2, expectedResult);
 
     const ResultObject result3 = QtConcurrent::blockingMappedReduced(
@@ -1090,13 +1097,14 @@ void testMappedReducedInitialValueThreadPool(QThreadPool *pool,
                                              InitialObject &&initialObject)
 {
     const ResultObject result1 = QtConcurrent::mappedReduced<ResultObject>(
-                pool, sourceObjectList, mapObject, reduceObject, initialObject);
+                pool, sourceObjectList, mapObject, reduceObject, initialObject).result();
     QCOMPARE(result1, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
-    const ResultObject result2 = QtConcurrent::mappedReduced<ResultObject>(
-                pool, sourceObjectList.constBegin(), sourceObjectList.constEnd(),
-                mapObject, reduceObject, initialObject);
+    const ResultObject result2 =
+            QtConcurrent::mappedReduced<ResultObject>(pool, sourceObjectList.constBegin(),
+                                                      sourceObjectList.constEnd(), mapObject,
+                                                      reduceObject, initialObject).result();
     QCOMPARE(result2, expectedResult);
     QCOMPARE(threadCount(), 1); // ensure the only one thread was working
 
diff --git a/tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp b/tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp
index 01b7c1b895..c4d7c1d362 100644
--- a/tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp
+++ b/tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp
@@ -127,14 +127,14 @@ void tst_QtConcurrentRun::runLightFunction()
 
     void (*f3)(QPromise<int> &) = lightOverloaded;
     qDebug("starting function with promise");
-    QFuture<void> future3 = run(f3);
+    QFuture<int> future3 = run(f3);
     qDebug("waiting");
     future3.waitForFinished();
     qDebug("done");
 
     void (*f4)(QPromise<double> &, int v) = lightOverloaded;
     qDebug("starting function with promise and with arg");
-    QFuture<void> future4 = run(f4, 2);
+    QFuture<double> future4 = run(f4, 2);
     qDebug("waiting");
     future4.waitForFinished();
     qDebug("done");
diff --git a/tests/auto/corelib/io/qlockfile/tst_qlockfile.cpp b/tests/auto/corelib/io/qlockfile/tst_qlockfile.cpp
index 54d2182e5c..431ca4dd5a 100644
--- a/tests/auto/corelib/io/qlockfile/tst_qlockfile.cpp
+++ b/tests/auto/corelib/io/qlockfile/tst_qlockfile.cpp
@@ -249,7 +249,7 @@ void tst_QLockFile::waitForLock()
     if (!releaseEarly) // only let the thread release the lock now
         semMainThreadDone.release();
 
-    QVERIFY(ret); // waits for the thread to finish
+    QVERIFY(ret.result()); // waits for the thread to finish
 }
 
 void tst_QLockFile::staleLockFromCrashedProcess_data()
diff --git a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
index 0faa8e0be7..18e11c35ad 100644
--- a/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
+++ b/tests/auto/corelib/thread/qfuture/tst_qfuture.cpp
@@ -99,6 +99,7 @@ class tst_QFuture: public QObject
 private slots:
     void resultStore();
     void future();
+    void futureToVoid();
     void futureInterface();
     void refcounting();
     void cancel();
@@ -109,7 +110,6 @@ private slots:
     void progressText();
     void resultsAfterFinished();
     void resultsAsList();
-    void implicitConversions();
     void iterators();
     void iteratorsThread();
 #if QT_DEPRECATED_SINCE(6, 0)
@@ -609,6 +609,19 @@ void tst_QFuture::future()
     QCOMPARE(intFuture2.isFinished(), true);
 }
 
+void tst_QFuture::futureToVoid()
+{
+    QPromise<int> p;
+    QFuture<int> future = p.future();
+
+    p.start();
+    p.setProgressValue(42);
+    p.finish();
+
+    QFuture<void> voidFuture = QFuture<void>(future);
+    QCOMPARE(voidFuture.progressValue(), 42);
+}
+
 class IntResult : public QFutureInterface<int>
 {
 public:
@@ -1067,25 +1080,6 @@ void tst_QFuture::resultsAsList()
     QCOMPARE(results, QList<int>() << 1 << 2);
 }
 
-/*
-    Test that QFuture<T> can be implicitly converted to T
-*/
-void tst_QFuture::implicitConversions()
-{
-    QFutureInterface<QString> iface;
-    iface.reportStarted();
-
-    QFuture<QString> f(&iface);
-
-    const QString input("FooBar 2000");
-    iface.reportFinished(&input);
-
-    const QString result = f;
-    QCOMPARE(result, input);
-    QCOMPARE(QString(f), input);
-    QCOMPARE(static_cast<QString>(f), input);
-}
-
 void tst_QFuture::iterators()
 {
     {
diff --git a/tests/auto/corelib/thread/qfuturewatcher/tst_qfuturewatcher.cpp b/tests/auto/corelib/thread/qfuturewatcher/tst_qfuturewatcher.cpp
index 027bd1313e..e83ea07dd4 100644
--- a/tests/auto/corelib/thread/qfuturewatcher/tst_qfuturewatcher.cpp
+++ b/tests/auto/corelib/thread/qfuturewatcher/tst_qfuturewatcher.cpp
@@ -864,7 +864,7 @@ void tst_QFutureWatcher::suspendEvents()
 
 void tst_QFutureWatcher::suspended()
 {
-    QFutureWatcher<void> watcher;
+    QFutureWatcher<int> watcher;
     QSignalSpy resultReadySpy(&watcher, &QFutureWatcher<int>::resultReadyAt);
 #if QT_DEPRECATED_SINCE(6, 0)
 QT_WARNING_PUSH