Add QFutureInterfaceBase::get() to retrieve internals of QFuture

For generic interop with other parts of Qt, we need to be able to
extract the type-erased internal storage of QFuture<T>. In particular,
QtWebChannel needs this to transparently support QFuture<T> as a method
return type.

Task-number: QTBUG-92903
Change-Id: I763f054656b8810d58720262d364baf42c47eb37
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/thread/qfuture.h

@@ -300,6 +300,8 @@ private:
     template<class U>
     friend class QFuture;
 
+    friend class QFutureInterfaceBase;
+
     template<class Function, class ResultType, class ParentResultType>
     friend class QtPrivate::Continuation;
 
@@ -430,6 +432,12 @@ inline QFuture<void> QFutureInterface<void>::future()
     return QFuture<void>(this);
 }
 
+template<typename T>
+QFutureInterfaceBase QFutureInterfaceBase::get(const QFuture<T> &future)
+{
+    return future.d;
+}
+
 Q_DECLARE_SEQUENTIAL_ITERATOR(Future)
 
 QT_END_NAMESPACE

src/corelib/thread/qfutureinterface.h

@@ -177,6 +177,9 @@ public:
     // ### Qt 7: inline
     void swap(QFutureInterfaceBase &other) noexcept;
 
+    template<typename T>
+    static QFutureInterfaceBase get(const QFuture<T> &future);  // implemented in qfuture.h
+
 protected:
     // ### Qt 7: remove const from refT/derefT
     bool refT() const noexcept;

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

@@ -163,6 +163,8 @@ private slots:
 
     void createReadyFutures();
 
+    void getFutureInterface();
+
 private:
     using size_type = std::vector<int>::size_type;
 
@@ -3521,5 +3523,14 @@ void tst_QFuture::createReadyFutures()
 #endif
 }
 
+void tst_QFuture::getFutureInterface()
+{
+    const int val = 42;
+    QFuture<int> f = QtFuture::makeReadyFuture(val);
+
+    auto interface = QFutureInterfaceBase::get(f);
+    QCOMPARE(interface.resultCount(), 1);
+}
+
 QTEST_MAIN(tst_QFuture)
 #include "tst_qfuture.moc"