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>

2 files changed, 15 insertions, 21 deletions

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