QMetaType: Support converting any QFuture<T> to QFuture<void>

QMetaType now implicitly knows how to convert any QFuture<T> to
QFuture<void> without needing to manually register a converter
function.

QtWebChannel will make use of this to transparently support QFuture<T>
return types.

[ChangeLog][QtCore][QMetaType] QMetaType now supports converting any
QFuture<T> to QFuture<void>.

Task-number: QTBUG-92903
Change-Id: Ied7e71be37c346cc3d2c274ffb0d91a6821ab4d4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qmetatype.h

@@ -1092,6 +1092,14 @@ namespace QtPrivate
         static bool registerConverter() { return false; }
     };
 
+#if QT_CONFIG(future)
+    template<typename T>
+    struct MetaTypeQFutureHelper
+    {
+        static bool registerConverter() { return false; }
+    };
+#endif
+
     Q_CORE_EXPORT bool isBuiltinType(const QByteArray &type);
 } // namespace QtPrivate
 
@@ -1218,6 +1226,9 @@ int qRegisterNormalizedMetaType(const QT_PREPEND_NAMESPACE(QByteArray) &normaliz
     QtPrivate::AssociativeContainerTransformationHelper<T>::registerMutableView();
     QtPrivate::MetaTypePairHelper<T>::registerConverter();
     QtPrivate::MetaTypeSmartPointerHelper<T>::registerConverter();
+#if QT_CONFIG(future)
+    QtPrivate::MetaTypeQFutureHelper<T>::registerConverter();
+#endif
 
     if (normalizedTypeName != metaType.name())
         QMetaType::registerNormalizedTypedef(normalizedTypeName, metaType);

src/corelib/thread/qfuture.h

@@ -43,6 +43,7 @@
 #include <QtCore/qglobal.h>
 
 #include <QtCore/qfutureinterface.h>
+#include <QtCore/qmetatype.h>
 #include <QtCore/qstring.h>
 
 #include <QtCore/qfuture_impl.h>
@@ -438,8 +439,27 @@ QFutureInterfaceBase QFutureInterfaceBase::get(const QFuture<T> &future)
     return future.d;
 }
 
+namespace QtPrivate
+{
+
+template<typename T>
+struct MetaTypeQFutureHelper<QFuture<T>>
+{
+    static bool registerConverter() {
+        if constexpr (std::is_same_v<T, void>)
+            return false;
+
+        return QMetaType::registerConverter<QFuture<T>, QFuture<void>>(
+                [](const QFuture<T> &future) { return QFuture<void>(future); });
+    }
+};
+
+}  // namespace QtPrivate
+
 Q_DECLARE_SEQUENTIAL_ITERATOR(Future)
 
 QT_END_NAMESPACE
 
+Q_DECLARE_METATYPE_TEMPLATE_1ARG(QFuture)
+
 #endif // QFUTURE_H

tests/auto/corelib/thread/qfuture/tst_qfuture.cpp

@@ -164,6 +164,7 @@ private slots:
     void createReadyFutures();
 
     void getFutureInterface();
+    void convertQMetaType();
 
 private:
     using size_type = std::vector<int>::size_type;
@@ -3532,5 +3533,22 @@ void tst_QFuture::getFutureInterface()
     QCOMPARE(interface.resultCount(), 1);
 }
 
+void tst_QFuture::convertQMetaType()
+{
+    const auto intType = QMetaType::fromType<QFuture<int>>();
+    const auto voidType = QMetaType::fromType<QFuture<void>>();
+
+    QVERIFY(QMetaType::canConvert(intType, voidType));
+
+    const int val = 42;
+    QFuture<int> f = QtFuture::makeReadyFuture(val);
+    auto variant = QVariant::fromValue(f);
+    QVERIFY(variant.convert(voidType));
+
+    const auto voidFuture = variant.value<QFuture<void>>();
+    QVERIFY(voidFuture.isValid());
+    QVERIFY(voidFuture.isFinished());
+}
+
 QTEST_MAIN(tst_QFuture)
 #include "tst_qfuture.moc"